Transactions of the Geological Society, 1st series, vol. 2/On certain Products obtained in the Distillation of Wood

On certain Products obtained in the Distillation of Wood
by John MacCulloch


of the


I. On certain Products obtained in the Distillation of Wood with some account of Bituminous Substances, and Remarks on Coal.

By J. Mac Culloch, M.D. F.L.S.

Chemist to the Ordnance, and Lecturer on Chemistry at the Royal Military Academy at Woolwich.

IT is well known that when wood and other vegetable substances are submitted to destructive distillation, there is produced, among other matters, a black dense fluid resembling molasses or common tar. My connection with the Ordnance powder-mills having compelled me to examine some of the properties of this substance, which is obtained in large quantities in the process of making charcoal, I was induced to extend my enquiries, in consequence of finding that the nature of the compound has not hitherto been understood.

As it is commonly called tar by the workmen, I shall use this term for want of a better.

This tar is very inflammable, and so liquid that it may be burnt in a lamp.

Although it appears to be an uniform fluid, it contains a great quantity of acetic acid, in a state of loose combination or mixture. For, by washing with water, a great part of this is separated; the water at the same time acquiring a colour from a portion of the tar which is retained in solution by the acid. Boiling water takes up a larger portion, and the tar acquires from this operation a thicker and more pitchy consistence.

Lime and the carbonated alkalies separate the acid with ease, carrying away also a portion of the tar which continues united to the solution. With subcarbonate of potash it thus forms in the first instance an uniform solution of a brown colour, but a continuance of trituration or boiling renders it pitchy and tenacious, after which it forms no further union with the mild alkalies.

It is perfectly and readily soluble in alcohol, in ether, in the pure alkaline lixivia, in acetic acid and in the mineral acids. The fat oils and the new essential oils dissolve only a small portion of it; but the drying oils and the latter when thickened by age act more readily. Coloured oil of turpentine dissolves a good deal of it. Naphtha hardly exerts any action, acquiring a scarcely sensible brown colour. If heat be applied to assist the solution, the portion taken up is deposited on cooling.

When it is subjected to distillation in a heat sufficient to keep it in a gentle ebullition, an oily looking matter passes over in considerable proportion, which sinks to the bottom of the water into which the tube is inserted. It is first of a pale colour, resembling oil of peppermint, but becomes gradually darker as the operation advances, till it acquires a deep brown hue.

If the operation be pushed by increasing the heat of the retort to redness, there remains at length only a mass of spungy charcoal, and the substance is totally convened into the following new compounds, namely, the residuary charcoal, the oily matter, and the matter held in solution in the water of the apparatus. This latter proves to consist of a large portion of acetic acid, with which is combined a very little ammonia.

There is no inflammable gas given out in this process unless the heat be carelessly managed. If the vapour of the oily matter as it arises be exposed to the sides of the retort elevated to a high temperature, it is decomposed, and instead of oil there are thus obtained by a violent distillation in a naked fire, scarcely any products but acetic acid and an inflammable gas. This fact is analogous to those occurring in the ordinary process for decomposing such inflammable bodies as can be made to put on the gasseous state─and we ought, in fact, to consider every process of this kind, where a rapid distillation with a hot fire is used, as a succession of decompositions; the matter first produced being afterwards exposed to another process of destruction. It is not therefore perhaps very correct language, to say that vegetables yield a great quantity of inflammable gas on distillation with a naked fire; this is the produce of a second distillation which by the common mode of operating is confounded with the first. As this reasoning applies equally to all other similar processes, it would be desirable to use a more accurate mode of describing this common operation by which we might in some important instances be led to a more correct practice. Thus, for example, in the common mode of distilling coal to produce the inflammable gasses, this double operation is carried on at once by the application of the petroleum and naphtha at first produced to the heated iron of the retort. It is in consequence of this imperfect mode of exposing the fluids thus generated to a second heat, that so large a portion of the petroleum is distilled unchanged. By causing it to pass a second time in contact with heated iron, while in the state of vapour, it may be resolved completely into inflammable gas and charcoal, and the produce of gas be thus considerably increased. This circumstance explains also the contradictory accounts given by different persons of the relative products of distillation, as applied to the various compound inflammables. To instance the case of camphor, which according to the mode of managing the process, may be caused to yield essential oil or inflammable gas or a mixture of both in various proportions. I need scarcely point out the advantages so obviously to be derived from this consideration in the economical process of procuring light from pit-coal, an operation at present conducted with less skill than it demands.

I distilled a portion of this tar in such a way as to obtain inflammable air only, and took the gas in five portions. The first burnt very faintly, the second rather better, the third and fourth portions with a good white flame, and the fifth burnt feeble and blue. No portion of it was equal in brilliancy of inflammation to the gas from pit-coal. On examination, it was found to contain much carbonic oxide, by which its nature, as far as it differs from the gas of coal, is readily understood. The cause of this difference will be apparent when the other circumstances in the constitution of this substance have been detailed. I thought it superfluous to examine accurately the nature of these gasses, but they probably consist of different mixtures of carbonic oxide, with light and heavy hydrocarbonate and olefiant gasses, if indeed, (as I much doubt) there be any real boundary by which the composition of these three last gasses can be defined.

If the process of distillation which I have now described be stopped when the oily matter begins to acquire a brown colour, and when the production of acetic acid is less perceptible, the matter in the retort will be found when cold, to have assumed a solid consistence. In this state it resembles either pitch or asphaltum, according to the degree of heat it has undergone after it became capable of solidifying.

I will describe this substance as it appears when it first becomes solid, the reason of which will soon be apparent.

Previously to its arriving at this state, it bears a considerable resemblance to maltha, being of a consistence intermediate between that of petroleum and asphaltum, but I did not completely examine its chemical properties in this condition because they appeared not to differ from what might be expected, and its history will be sufficiently full without it. In the solid state it is brilliant and shining and breaks with a conchoidal fracture and some external resemblance to obsidian. It has a pungent burning taste and the well known smell of wood smoke. It is heavier than the specimen of asphaltum with which I compared it, having a specific gravity of 1.254, while that of the asphaltum was 1.202. It is fusible and readily inflammable, burning with a white flame. It is electric and exhibits the same electricity as the resinous bodies. When heated in an open vessel, it smokes, and if kept in fusion till it ceases to smoke, it at length ceases to be fusible and is ultimately converted into a coal. During this progress it becomes more brilliant and less fusible, its fracture also from conchoidal becomes more splintery, and it puts on the appearance of asphaltum so accurately that the eye cannot detect the difference. Its specific gravity also diminishes, and its chemical properties vary in the way I am now about to detail.

I have described the perfect solubility of the tar in alcohol. The softest specimens of the pitch are nearly as soluble, leaving only a small residuum, which is infusible and powdery. The harder specimens become in proportion less soluble, and leave a larger residuum; and those which have been the longest exposed to heat scarcely give a stain to the alcohol, resembling in this respect the driest specimens of asphaltum. The analogy is here very apparent, for asphaltum may approach more or less to petroleum, and the various specimens of it are found to exhibit various degrees of solubility in alcohol. That which is least fusible in the fire, is, in both cases, the least soluble in alcohol. And by this consideration, the jarring accounts which have been given of the solubility of asphaltum in alcohol may be reconciled, and it will be seen in the sequel, that the history of this substance illustrates, in every respect, the true nature of the several varieties of the bitumens, substances whose mutual relations, and the causes of whose chemical diversity have hitherto not been understood.

If a perfectly soluble specimen be dissolved in alcohol, it is obtained unchanged by evaporating the spirit. In any other case, the matter which the alcohol has taken up is precisely similar to the pitch in its first state, and the residuum resembles that which is the result of fusion when it refuses longer to melt. Alcohol therefore separates the pure pitch from that, which by a process of decomposition has been nearly carbonized. Ether acts upon this substance as readily and in the same manner as alcohol does. In lixivium of pure potash it is more completely soluble than in alcohol, and forms with it an intensely brown solution which is diffusible in water without change, and which, on the addition of an acid, deposits the matter in a powdery form and apparently unchanged. It is also soluble in water of ammonia with similar appearances. It is scarcely soluble in the pale oil of turpentine, but more readily in the darker. It is slightly soluble in the fat oils, in tallow and in wax, but is considerably more soluble in drying oil. In all these cases its solubility varies, from the same causes as those which affect its solubility in alcohol. Naphtha, whether pale or brown, has no action on it when cold and takes up but a very minute proportion even with the assistance of heat. It fuses into an uniform mass with sulphur, with resin and with asphaltum.

Acetic acid, which dissolves so many of the compound inflammables, effects a compleat solution of it and in large proportion, and this compound is precisely similar to the empyreumatic acid as it proceeds from the iron retorts in which the charcoal is distilled. It would be desirable in an economical point of view, to discover a method of freeing the acid from the pitch. After many trials, by combining the foul acid with various bases and again separating it, it was always found to retain the overpowering smell of wood-tar. If the acid is combined with the pitch at a high temperature, a large proportion of it separates in the form of tar on cooling. Muriatic acid, after long boiling on the pitch, became brown and dissolved a little of it.

By digestion with sulphuric acid it was dissolved forming a brown oily looking fluid, sulphurous acid being at the same time disengaged. By dilution with water, a smell resembling peppermint was produced, as happens in a similar case with camphor, and the pitch was thrown down. The action of the red nitrous acid on it is violent; the acid is decomposed with great ebullition and a portion of the pitch is converted into coal. In diluted nitric acid it dissolves and produces an uniform brown fluid. On continuing to apply nitrous acid according to the process of Mr. Hatchett, solutions similar to those which he has described as having been obtained from the resins and bitumens, are produced.

I exposed a quantity of the pitch to a careful distillation through water. As might be expected from what I described before in the distillation of the tar, this process gave results nearly similar to the former. The oily matter differed in being of a brown colour and in having a greater specific gravity, and much less acid was produced; the residuum was charcoal. The whole process of distillation appears, therefore, to be a decomposition by which the pitchy substance is converted into oil, acetic acid, ammonia and charcoal.

I proceeded next to examine the oil. It has a violently pungent taste and smell. It is scarcely heavier than water; so that it sinks in that fluid with difficulty, leaving generally some drops on the surface. It is perfectly soluble in alcohol, in ether, in caustic alkali, in olive oil, and in linseed oil. It will unite neither to naphtha, nor to the recent essential oils, but is soluble in the old ones. From these properties, it belongs to the class of the essential oils, but exhibits at the same time other qualities by which it is distinguished from the whole of them.

Having thus examined the most remarkable chemical properties of this substance, it will not be irrelevant to point out its differences from and its analogies with those substances which it most resembles, namely resin and the bitumens. Resin, as is well known, is eminently soluble in all the substances in which this is dissolved, and also in those with which this refuses to unite, even naphtha. But the general analogy between essential oil, turpentine and resin, is so close to that of the three substances which I have described, that it will not perhaps be superfluous here to make some remarks on the nature of common resin and the substances connected with it, pitch, tar, turpentine and essential oil, as their history will also illustrate that of the substance I am describing, and as it appears, like that of the bitumens, to have been somewhat mistaken.

If turpentine, as it flows from the fir in a liquid state, be exposed for a considerable time to the action of the atmosphere, it becomes brittle, and is converted into resin, in consequence as it is supposed of the absorption of oxygen. If the same turpentine be exposed to the action of the fire, a colourless volatile oil is separated, and resin remains in the retort. This however is not a mere case of the separation of a more volatile from a fixed substance, for a decomposition takes place, and acetic acid is generated. Nor can turpentine be again reproduced by mixing together the essential oil and the resin—it then forms a varnish. The essential oil is in fact a new compound, produced from the vegetable elements by the action of fire; and although properly enough classed with those essential oils which are vegetable secretions, differs from them in some of its chemical properties. It is, for example, difficultly[errata 1] soluble in alcohol, but on exposure to air it becomes thick and yellow, and is then easy of solution in the same substance.

If the resin, which is the residuum of this distillation be still further heated, it gives over a thick and high coloured oil, gradually increasing in weight, till it equals, and at length exceeds the specific gravity of water. The residuum becomes ultimately black, and very brittle, remaining soluble in ether and in lixivium of potash, but refusing to dissolve in alcohol.

Common tar differs from turpentine in containing a portion of the vegetable tar now under review, mixed with common turpentine and with the acetic acid which is formed in the distillation to which the wood is subjected for the purpose of obtaining it. Evaporation converts this into pitch, by decomposing it.

In this process, an essential oil, compounded of the oil of turpentine and the oil of wood, together with a portion of acetic acid, is separated, and the residuum or common pitch, is a compound of resin and the wood pitch which I have been describing. To this admixture, and not to that of adventitious charcoal produced in combustion, is the black colour of common pitch owing.

The analogy between this wood pitch and the bitumens is equally striking, and the preceding history of these compounds will throw light on the several varieties of the bituminous substances.

Assuming the tar as the medium form, it is seen that when exposed to heat it gives over oil, and that pitch remains. Thus, petroleum yields naphtha and asphaltum; and thus too, asphaltum exhibits all the gradations which I have described in the pitch, in properties varying in a similar manner, according to its particular state. In the process of distillation the principal difference will be found to consist in the relative quantities of acetic acid and ammonia, which they severally yield; the former chiefly characterizing the wood tar, and the latter the petroleum. From the same chemical cause which produces this effect arises also the difference in the nature of the inflammable gasses which are produced from these different substances.

The sensible qualities of the bitumens (their taste and smell) are in all states utterly and entirely different from those of the vegetable tar. Petroleum is also much less soluble in alcohol, and further differs from the vegetable tar in being perfectly soluble in naphtha. In their solubilities in oil of turpentine they resemble, each other, as well as in their habitudes with acetic acid and the alkaline lixivia, although the vegetable tar will be found the more readily soluble of the two. I need not repeat the circumstances in which the essential oil of wood differs from naphtha. It is a sufficiently characteristic one, that it forms no union with this latter.

It has been already shown that the difference between the pitch and asphaltum is considerable, when the former is in its first state, particularly with regard to its solubility in alcohol.

But if we compare the most brittle specimens of the pitch with common specimens of asphaltum, the differences, except as far as smell and taste are concerned, are not so apparent, and the reason of this will be obvious on considering their fundamental similarity of composition. The chief ingredients of both are carbon and hydrogen. By the application of heat, the proportions of these substances are altered in both cases, the hydrogen being abstracted in the greatest ratio, to form the new compound (the oil) in which hydrogen predominates. The ultimate result of both is charcoal. Asphaltum will be found to combine pretty nearly in the same way, with all the substances I have above enumerated as combining with the pitch. Its essential difference however consists in its solubility in naphtha, and by this test they are readily distinguished.

The chemical difference to which these different properties of substances so similar are owing, will be evident on considering some of the circumstances before related. The disproportion of acetic acid and carbonic oxide produced from the wood pitch, when compared with the produce of the bitumen, proves that it contains oxygen and azote in proportions different from those in which the same substances exist in the bitumens; and that in particular it contains a considerable quantity of the former. The result would not repay the toil required to investigate these proportions, which are probably also subject to considerable variation.

It is obvious that this substance is a new compound, formed by the action of fire on vegetable elementary matter; but all that we can determine of its nature is, that in conformity to modern chemical nomenclature, it is formed of carbon, hydrogen, oxygen, and azote. The carbon and hydrogen constitute its basis, as they do that of the bitumens, and the large proportion of oxygen appears to give it the peculiar properties by which it is distinguished from them It seems evident however, that no very great change is wanting to convert the one of these into the other.

The question so much agitated, of the conversion of vegetables into coal, would appear to receive some illustration from the history of the compound which I have been describing, and since (as I shall by and by show) it has actually been confounded with bitumen, and has been adduced as an instance of the artificial production of coal by the action of fire, I shall make no apology for pursuing this subject. Indeed the general chemical resemblance between the mineral bitumens and this vegetable bitumen, if it may be so called, is so striking, that we may, at first sight, be easily led to suppose that the same agent has produced both, and excuse the mistakes which seem to have occurred on this subject. But a cursory view of the several substances which have been classed under the head of bitumens, may enable us to form a clearer notion of the limited extent of this analogy, at the same time that it will perhaps assist us in correcting some errors which have crept into our arrangements of them.

It is necessary to separate from the bitumens three or four mineral substances, which differ completely both in chemical and ordinary characters, but which are approximated to each other by some general resemblance. These are, amber, mellilite, and the subterraneous resins of Cologne, Bovey, and Highgate. The two first are more nearly associated by the property they have of yielding a peculiar acid; and of the three last, it may perhaps be fairly doubted, whether they are more entitled to be ranked among the mineral substances strictly so called, than the other vegetable matters which are found in alluvial soils.

The nature and relations of naphtha, petroleum, maltha, and asphaltum, will, I trust, appear sufficiently clear from what I have above related, but I cannot forbear remarking on some false hypotheses which have been held respecting these substances, and their relation to other bodies. It is evident, from considering the products of their decomposition, that the basis of naphtha and of all the intermediate stages of bitumen, down to asphaltum, are carbon and hydrogen, modified by certain small proportions of oxygen and azote. It is in the relative proportions chiefly of these two ingredients that naphtha differs from petroleum, petroleum from maltha, and maltha from asphaltum. If we distill either of these more solid substances with a very gentle heat, we obtain naphtha, in which the proportion of the hydrogen to the carbon is increased to a maximum ratio. If the heat is greater, we obtain a substance of a darker colour, in which that ratio is less; and, for this reason, the distillation of asphaltum affords a darker oil than that of petroleum, because its composition cannot be dissolved but in a higher temperature.

For the same reason also petroleum is easily rectified into naphtha. Asphaltum, in its ordinary state, contains the two ingredients in a ratio in which the carbon bears a large proportion to the hydrogen, and that ratio is reduced to the minimum, or becomes evanescent, when by the continuance of distillation, charcoal alone remains behind. A large portion of the oxygen, and also of the azote is disengaged during this process, but not the whole, since the darker compounds still give it over on repeating the process. The naphtha is probably entirely exempt from oxygen. With this view we cannot accede to the notion, that the absorption of oxygen is capable of converting naphtha or petroleum into asphaltum; or that the harder bitumens originate from the oxygenation of the more liquid. It is more consonant to the nature of these substances to suppose, that the change consists in the alteration of the relative proportions of the hydrogen and carbon, but whether this is performed by the action of heat, or of other causes volatilizing the hydrogen, or by the contact of oxygen converting it into water, cannot now be determined by any facts that we are acquainted with. Experiments on the induration of the essential oils may throw some light on this question. It will here perhaps be remarked, that there is a difference in the substances as they are produced artificially by the distillation of coal, and as they are found in nature. Thus, for example, the artificial petroleum of coal differs from that of nature, in being much more soluble in alcohol. Yet, this circumstance may arise from the insensible gradation of difference which I have above remarked in the similar compounds, and thus in the series of gradation, specimens absolutely corresponding, whether artificial or natural, may exhibit the same chemical characters.

Thus, as I have shown that there is a sort of gradation from naphtha to asphaltum, through a series of undefinable petrolea, so this analogy may be extended to the next general variety of the bitumens, coal.

The several varieties of coal are supposed to consist of charcoal and asphaltum, or of charcoal and bitumen, combined in as many different proportions. Charcoal is undoubtedly found mixed with coal, but it does not appear correct to consider pit coal as either a mixture or combination of any bitumen with charcoal. The action of naphtha on its varieties, often none and always sparing, shows that bitumen does net exist in it in a mixed state. It will be more consonant to the analogies of the other bituminous substances, to consider coal in its several varieties as a bitumen, varying in its composition, from the fattest specimens of Newcastle to the driest of Kilkenny, and owing its compactness, as well as the other modifications which it exhibits, to the peculiar circumstances under which it has been formed, the changes it may subsequently have undergone, or the substances with which it has accidentally been mixed. The power of yielding naphtha on distillation, is rather to be resorted to as the distinction between the one end and the other of the series; and it would be surely equally correct to call coal a compound of charcoal and naphtha, as a compound of charcoal and asphaltum.

Its several varieties will moreover be found to vary from each other by containing greater or less proportions of carbon, compared with their other ingredients; just as in asphaltum the relative proportions of the hydrogen, azote, and oxygen, to the carbon, are found to differ from those which constitute petroleum or naphtha.

The last link of the chain of coal (chemically considered) is anthracite, which contains only carbon, if we reckon the earths mixed with it as adventitious matter. So the last result of the distillation of asphaltum is charcoal, and the intermediate steps through which asphaltum passes in its progress to charcoal, resemble precisely the corresponding changes which occur in the distillation of coal till coak is formed, and confirm by their chemical analogy the view here held forth of the chemical composition of coal, and the gradation to be traced in nature from fat coal to anthracite. If asphaltum be subjected to distillation it gives petroleum. By degrees its solubility in naphta diminishes, in consequence of its carbonaceous ingredient becoming more disproportioned to its hydrogen. At a particular period of this distillation it will be found to resemble fat coal; by and by, it resembles blind coal, and gives no stain to naphtha; ultimately, pure charcoal remains. All these bituminous compounds may therefore properly be said to belong to one genus or family, of which the principal chemical ingredients are carbon and hydrogen, and it is chiefly to the difference in the relative proportions of those two substances that we are to look for the differences which characterize the several bitumens, from naphtha placed at one extreme, to anthracite placed at the other. The chasm in this series from asphaltum to fat coal, is in fact, rather apparent than real, being more properly a mechanical or accidental, than a chemical or essential one. I cannot here avoid taking notice of the very loose experiments of Mr. Kirwan on the analysis of coal, (which consisted in projecting portions of coal on melted nitre) as his deductions are at war with this view of the subject, although not more so than with all chemical reasoning. They were founded on an assumption, that coal was carbon impregnated sometimes with maltha and sometimes with asphaltum—a distinction quite unnecessary if the supposition were true. If we conceive coal to be compounded in this way, it would be more obvious to consider it as formed of carbon and petroleum, since by a regulated heat it can be separated into those two substances. The theory of the experiment is equally assumed and the conclusions equally groundless, when it is inferred that of this compound, (coal) the carbon alone possesses, the power of decomposing the nitre, and that the proportions of these supposed ingredients may thus be determined. The varying temperature of the nitre, would necessarily produce considerable variations and uncertainty in its action, and in the consequent accuracy of the results; but it is plain, that the effect of this contrivance was to separate by a sort of distillation the petroleum which fire elicits from coal, and that the method could neither be so accurate as that of ordinary distillation, from the greater irregularities to which it was subject and the difficulty of conducting it, and that it proves nothing with regard to the composition or nature of coal. In the examination of maltha, and asphaltum, the defect of this method is still more apparent. If heat and flame be applied to these bitumens, with, access of air, they are either consumed without leaving any carbon, or that carbon which there is not oxygen enough present to burn, is deposited in a state of very minute division in proportion as it is volatilized, during the formation of the naphtha or petroleum, the more hydrogenous part of the compound. But if this part is separated without flame, either by a more moderate heat, or by excluding oxygen from it, the carbon is rendered apparent by its affinity of aggregation, which causes it in the end to assume comparatively refractory powers, and a more solid form.

Such are the views I would entertain of the bituminous genus, in which as it is found in Nature, all traces of organization or resemblance to vegetable and animal inflammable matter, have so thoroughly ceased, that we are entitled to give its several species a fair rank among minerals. But there is yet another division of inflammable and subterraneous substances connected with these, of which the claims may appear doubtful. Retaining as they do, the traces of organization, and that sometimes in great perfection, it may be often questioned whether they do not more properly rank with the fossil remains, than with the minerals properly so called. They are well distinguished by the name of Lignites. At one end of this series is placed jet, in which the traces of vegetable origin are nearly obliterated. Surturbrand and the several varieties of brown coal, including Cologne earth, connect it gradually with submerged wood and peat. The experiments I have already related prove that the substance resembling bitumen, which is produced by the action of fire in the ordinary way on vegetables, differs from it essentially, and it has been seen that solubility in naphtha is the readiest criterion by which these substances can be distinguished. To assure myself of the accuracy of this test I mixed the petroleum of coal with the black oil of wood in several proportions, and by the application of naphtha separated the one from the other. By this simple method therefore I expected to detect not only the progress of bituminization from simple turf to jet, but to assure myself whether in the examinations hitherto made by others of these different substances, any mistake had arisen from confounding the vegetable bitumen with true bitumen when distillation was used to investigate their nature.

Vegetable turf in all its varieties, as well as brown coal, gave a considerable colour to lixivium of potash, but the same menstruum produced no effect on jet, or surturbrand. Nor had naphtha or alcohol any action except on the resinous lignite of Bovey, from which they extracted the resinous matter which that variety contains.

I therefore subjected these different substances to distillation, trusting that by the produce, I should ascertain, not only the fact, but the progress of bituminization.

Submerged wood, from peat mosses in Cumberland, gave a brown oil, smelling of the wood tar and refusing to dissolve in naphtha. In this case therefore, no appearance of a change towards bitumen was exhibited. A compact pitchy looking peat gave an oil which had a fetid smell, neither resembling that of wood tar, nor bitumen, and which was very slightly soluble in naphtha.

The Bovey brown (board) coal, produced an oil of a peculiar smell, but most resembling that of wood tar, and much more soluble in naphtha than the preceding. Having a larger quantity of this, I separated the soluble part by naphtha, and in the remainder, or insoluble oil, the smell of wood tar was powerful, notwithstanding the strong odour of the naphtha. Here then the progress of bituminization had advanced another step. The resin of this wood, on which a particular name has lately been bestowed, I consider as an adventitious and accidental substance, and the natural produce of the tree, now probably unknown, which occupies these alluvial strata, as other lost productions of Nature are detected in other alluvial soils.

A specimen of black lignite from Sussex gave an oil which resembled the former in smell, and perhaps did not differ much from it in its solubility in naphtha, but I had not enough of the substance to institute an accurate comparison, neither in fact, could it serve any purpose. A similar substance from Bovey gave similar results.

The oil which was distilled from jet was of a greater specific gravity than any of the preceding, and smelled strongly of petroleum. It seemed to be soluble in naphtha as readily as the specimen of petroleum with which I compared it. Indeed had it not been that a greater quantity of acid was given over in this process than from any of the varieties of coal, I know not that any chemical distinction between the two would have existed. The mineralogical one is still considerable. The several specimens above enumerated, yielded each a large portion of acetic acid, marking as clearly as the peculiar sort of oil did, the remains of unchanged vegetable matter.

Examining therefore the alteration produced by water on common turf, or submerged wood, we have all the evidence of demonstration that its action is sufficient to convert them into substances capable of yielding bitumen on distillation.

That the same action having operated through a longer period has produced the change in the brown coal of Bovey, is rendered extremely probable by the geognostic relations of that coal. From this to the harder lignites, surturbrand and jet, the transition is so gradual, that there seems no reason to limit the power of water to produce the effect of bituminization in all these varieties, nor is there aught in this change so dissonant from other chemical actions, as to make us hesitate in adopting this cause. In the ordinary process of vegetable putrefaction and destruction, a variety of compound gases are formed by the reaction of their elements, and carbon alone, or rather carbon united to a portion of hydrogen, remains behind. Here the oxygen is completely dissipated, together with the azote, and the greater portion of the hydrogen. Analogous circumstances determine the putrefaction of animal matter, but in this case the play of affinities is so intricate, that a large portion of the carbon is volatilized in the gaseous form. By the constant affusion of water however, this process may be so modified, that the greater part of the hydrogen and carbon will be retained, and enter, together with minute portions of other gases, into a new compound resembling fat, which has obtained the name of adipocire. The analogy is strong, and the gradual deoxydation of the wood in this process is visible in the different stages of bituminization.

Such, as far as observations have yet gone, is our knowledge of this process and of the power of water in producing it. To repeat such an experiment in the laboratory seems impossible, since the necessary element of time must be wanting to complete it. But the action of fire being of shorter duration, and affording us also readier means of imitating Nature in those operations in which she has wrought with the same agent, it is worth our while to consider, if by it we can produce from vegetables the bituminous matters under review. It is not necessary to say how intimately this question is connected with our speculations on the origin of coal, since Sir James Hall's experiments were expressly intended to illustrate this view of the subject. In this, it is related that “coal” was produced, from “fir saw dust” by the usual method employed in these experiments, and that pieces of wood were changed “to a jet-black and inflammable substance, generally very porous,” in some specimens of which “the vegetable fibres were still visible.” There is no reason to doubt that the substance produced in these experiments, was that black matter which I have described in the first part of this paper, which, however resembling bitumen in colour and inflammability, I have proved to be a different substance, and that the igneous theory of the origin of coal will receive no support from them, as far at least as relates to the conversion of vegetable matter into bitumen. I need take no notice of the modifications derived from a mixture of animal matter in these experiments, as it is not my desire to enter into a discussion of the general question, but to state such chemical facts as arose in the experiments I undertook. And since it is certain that vegetables alone are competent to the production of bitumen, and that the geological history of coal does not justify a supposition that animals have been concerned in its production, it is perhaps unnecessary to investigate that question further.

To satisfy myself whether any essential chemical difference would result from the experiments performed by simple heat, and those performed by heat under pressure, I repeated these trials, by heating wood in close gun barrels, introducing occasionally lime, clay, or other matters to absorb the acid generated, and give the greater chance for the disoxygenation and bituminization of the wood. But the produce only differed from that of the experiments in open vessels, by the circumstance which is mentioned in Sir James Hall's, paper; namely, the mixture of a porous charcoal, or a half destroyed vegetable structure. In all cases the bituminous looking matter was vegetable tar, not bitumen.

Thus far then perhaps we are justified in concluding that the action of water, and not that of fire, has converted the vegetable matters into bitumen. It is another question to determine how that bituminous matter in its several forms of peat or lignite, has been converted into coal, into a substance differing mechanically, rather than chemically from it, if, without misleading, I may use the contrast of these terms.

There is a wide interval between the external characters of the lignites and of coal, and though we cannot presume to state the period which Nature has used in her operations, nor during how long a space the causes have continued to act, before the vegetable matter has undergone its ultimate change into coal, nor therefore whether the long continued agency of water and pressure may not have produced the required changes, yet, since Philosophers of high reputation have supposed that fire has been a probable cause of this conversion, and that this theory is supported by considerable evidence in some analogous cases, it is our duty to examine by experiment, what effects conducing to this end may result from our limited trials. The foregoing experiments show that the fire of our furnaces does not convert wood into bitumen, and the processes of Nature seem to prove that water can produce this effect, and that jet, the bituminous lignite which approaches nearest to coal in its chemical characters, is the result of this action. Yet there is an interval between jet and coal as I have already observed, requiring explanation. The chemical characters may be identical, but the mineralogical resemblance is still wanting. It is possible that the agency of fire may account for this ultimate change, and that its action on beds of lignite and peat has converted not wood but vegetable matter already bituminized by water into coal. Pursuing this train of investigations I was induced to try if jet, the most perfectly bituminized lignite, could by the application of heat under pressure be converted into coal. For this purpose I introduced powdered jet into gun barrels, placing it between two portions of rammed Stourbridge clay, with the view of absorbing a part of the distilled petroleum when it might be formed in greater quantity than was requisite for the success of the experiment, and where by its conversion into hydrogen, it might endanger the bursting of the apparatus. The barrels, which were Swedish, were held in a moderate red heat till they burst, when they were instantly withdrawn and cooled in water to prevent the further volatilization of the bituminous matter. As the opening was generally no larger than a pin hole there was no difficulty in cooling the apparatus in time. In this way, among some failures, I procured a perfect fusion of the jet, which exhibited the true characters of coal, and was taken out with the impression of the irregularities in the barrel. I need not add that in this case the produce had not merely the colour and inflammability but the fracture of coal and its odour on burning. It is not unlikely that by a sufficient repetition of these experiments with better regulated heats and more leisure than I possessed, several varieties of coal might have been in this way produced. Indeed some of the specimens exhibited a dry, and others a fat appearance, but it was impossible in general to detach them from the barrels without reducing them to small fragments. Two other circumstances occurred deserving of notice. In one or two cases where the heat had been too great, a portion of the jet was reduced to charcoal, which continued attached to the coaly matter, and the clay was in every instance blackened to a considerable distance from the jet, and converted into a hard compact substance resembling bituminous shale in its smell and consistence.

Reverting to the chemical nature of the other lignites, there is very little reason to doubt that those among them which approach the nearest to a state of perfect bituminization, would have given results nearly similar, but I could not pursue the investigation for want of sufficient specimens. From peat we should expect but a mixed matter, varying between the bitumen of wood and true bitumen, according to the degree of change previously undergone; for that the process of bituminization is the effect of water, and not of fire, is rendered probable, as much by these trials as by the geological observations above mentioned. The conversion of bituminized wood into true coal may possibly be the effect of a consolidation produced by the agency of fire, but I shall leave this argument in the hands of those who have undertaken the defence of this theory—having entered into this train of reasoning, not by design, but from the unavoidable concatenation of experiments.[1]

A circumstance occurred in the coaly residuum of the wood tar which it is worth while to notice, although of an accidental nature, and not essentially affecting the history of the vegetable bitumen or pitch which I have described. It bore no resemblance to common charcoal, but was more like black lead. It was as glossy, and although not so soft, marked paper with a similar streak. It was inflated, and therefore minutely scaly, and porous, and was attracted by the magnet. Muriatic acid took up a portion of iron from it, as it does from many varieties of plumbago, and the remainder resembled plumbago after it had been submitted to the action of acids.

It was also exceedingly difficult to burn, requiring a long continued red heat, after which it left an oxide or rather a carbonat of iron, such as remains from the combustion of plumbago. It is in fact to be considered as an artificial plumbago, a substance of whose nature all the charcoals of difficult combustibility partake, deriving their resemblance apparently from the same cause.

The formation of this plumbaginous substance serves to shew a very powerful affinity between iron and carbon, even where the proportions are very different from those which enter into the composition of steel. But to effect this combination, it is necessary that the carbon be in a state of previous union with other substances, and that it be applied to the iron in that state. It will be in vain that we attempt to combine iron with charcoal for this end, unless the charcoal or carbon be in that state of very minute division in which it exists when precipitated by a new affinity from some previous combination.

It is necessary now to account for the iron in this compound.

This distillation of wood for charcoal is carried on in iron vessels, and hence is derived the iron which enters into the composition of the pitch. I will not say that it is solely derived thence, as it is probable that if there were iron contained in the vegetable matter, it would also be found in the same place. When the acetic acid has been separated the iron remains united to the pitch. This fact may shew us, that if in the destructive analysis of vegetable (and probably animal) matter, we trust to find the iron they may contain in the residual matter of the distillation, we may be disappointed, since it may be carried over, together with the substances I have now been describing, in the act of ebullition, as happens in this very case, its tendency being to combine with them, in preference to the charcoal.

As it was no part of my design to examine the vegetable elements, I did not pursue any experiments with this substance distilled in earthen vessels so as to ascertain whether in this case also it would contain iron, but I did enough to satisfy myself that the pitch was essentially the same in which ever way produced.

It is already known that a substance resembling plumbago is formed in water, it having been discovered by Fabroni in the country round Naples. It is equally known to be formed in the iron foundries, and the advocates for the igneous origin of coal have also contended for that of plumbago, and have supposed it to have been produced by the contact of melted greenstone with beds of coal. But even if we admit this cause of its formation, something else seems necessary for the production of the substance, and some other mode of applying the heat required before it can be produced. Nor indeed does the explanation sufficiently correspond with the general geological position of plumbago.

In numerous trials to combine iron with charcoal so as to form this substance, I have uniformly failed of success, except where as in the case above related, the charcoal or carbon has been in a state of previous combination, or was actually held in solution. In many trials on this principle, the results have been tolerably successful. If therefore we are to adopt an igneous theory of the formation of plumbago, it will be as easy to suppose that the action of subterraneous fire on mixtures of bitumen and iron has produced the compound of charcoal and iron, on the principles I have described, and this supposition will be more consonant to the chemical facts. But we are too little acquainted with the geological relations of plumbago to lay much stress at present upon this or any other hypothesis. It is evident that plumbago may be a produce of art, and could it be produced in as solid and compact a state as Nature affords it, the discovery would form a material addition to those useful ones for which the arts have been indebted to chemistry.

As nothing tends more to confusion of ideas than confusion of terms, I may be excused for proposing a name to the pitch of distilled wood, a name in familiar use, though hitherto unappropriated by chemists. It is in fact that which is well known to painters by the name of Bistre, although the nature of bistre has I believe never yet been examined; and the importance of it to the arts of design induces me to extend this article for a few lines. According to Dr. Lewis, bistre is produced from the soot of all wood, other receipt books give us the same account, but limit the sort of wood to beech without seeming aware of its real nature; but the colourmen use the soot of all wood indiscriminately.

Those artists who have made the tour of the highlands of Scotland, are well acquainted with that variety of it which varnishes the interior of a highland cottage.

In all these cases it is a very variable article, and the colour-maker being unacquainted with its real nature, is unable to rectify its faults, in consequence of which it is often unfit for use, notwithstanding the various operose and mysterious purifications it undergoes in his workshop. The causes of these varieties will be very evident to those who have read the foregoing experiments. An imperfect separation of essential oil and a consequent tenacity arising from its too near alliance to the tar, will appear to be its most common vice, and it is this which gives it that disagreeable gumminess and disposition to return to the pencil which is destructive of its best qualities. At times also from the same causes it is offensively yellow. So valuable is a brown colour that will work freely and with transparency, that the artists will be much obliged to him who shall render bistre equal in freedom and force to seppia. By distilling or evaporating the oil from the pitch, according to the process described above, a colour may be produced varying in tone from the warmest bistre brown down to black. At the same time the substance loses a great portion or the whole of its disagreeable tenacity, according to the degree of boiling it has undergone. By treatment in alcohol, results in some measure similar are produced, and the residuum of this solution is equal in colour to seppia, and totally void of tenacity. In either or both of these ways may the quality of this colour be improved.

It might perhaps be a matter worthy of trial, whether useful varieties in colour and quality might not be produced by the distillation of different woods. That which I used was procured either from willow or alder—the two woods chiefly used in the royal powder mills, but I cannot ascertain from which of them. The solution in lixivium of potash or of soda, a substance analogous to the resinous soaps, answers the purpose of ink, possessing a colour sufficiently intense and flowing freely from the pen without requiring gum. As it is indestructible by time, by the common acids or by the alkalies, perhaps it may be found a valuable substitute for this useful but fugacious substance. The compound of bistre and soda appears peculiarly well fitted for drawing in monochrome, since as it does not consist of a powder suspended in a vehicle, it is free from the peculiar defects, so well known to artists, which occur in colours thus compounded.

I may also add that it forms a substitute for asphaltum in drying oil where such a coloured varnish is wanted, and that it makes a very good japan varnish for metal if dissolved in spirit of wine, and heated strongly after its application. It is for practical men to see whether by combining it with asphaltum, lac, or the gums, some more useful and cheap compounds of this sort may not be produced.

  1. That I may not interrupt the text, I will add, in a note, a cursory account of the black matter which is deposited in bogs, and which seems to be the substance giving the pitchy appearance to the more compact varieties of peat. I have not seen it in the soft state in which it is first procured.

    When dry, it is black, sometimes dull, sometimes with the lustre of asphaltum. It is heavier than water. It is not electric. It is brittle, and breaks with a fracture intermediate between the splintery and conchoidal, resembling asphaltum generally in its external characters. Exposed to a red heat it is incinerated, giving a smoke possessing a modified smell of vegetable (pyroligueous) acid. It is not acted upon by boiling alcohol, ether, or naphtha; and in this latter circumstance, its difference from asphaltum is marked. Neither is it soluble in boiling water. It is readily dissolved in lixivium of potash, and by nitrous acid. It appears to be formed of the vegetable elements in the state of transition to bitumen, the carbon having been first held in solution, as it is in the water of dunghills, by the other matters with which it was combined, and being at length consolidated by the dissipation of a portion of them. The produce of its combustion shows it is combined with both hydrogen and oxygen.


  1. Original: difficulty was amended to difficultly: detail