Popular Science Monthly/Volume 58/December 1900/Oxygen and the Nature of Acids

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Erected in Paris by International Subscription.





[These selections from Priestley's account of the discovery of oxygen and from Lavoisier's first formal presentations of his theory of acids are classical examples of scientific work which will always be worth reading. They have also the historical interest due to the fact that the discoveries they describe served as the turning-point of chemistry to the paths it has since followed. The dates of publication were respectively 1775, 1776 and 1777. We realize the progress of the century when we remember that these experiments are now among the first in an elementary course. These two papers are also representatives of two well-defined types of scientific advance; Priestley's discovery was one of the happy accidents that often reward the investigator, one of the cases where he reaps a hundred fold, while Lavoisier's work was the result of gifted insight and careful consideration of the entire range of phenomena concerned. Lavoisier had, as is shown in this paper, the faculty of giving the right meaning to the data acquired by others. The phlogiston theory is now so much a matter of antiquity that it seems proper to give the modern equivalents of some of Priestley's terms: Air is used by him in the modern sense of gas, dephlogisticated air=oxygen, inflammable air=hydrogen, phlogisticated air=nitrogen, marine acid air=hydrochloric acid gas, fixed air=carbon dioxid, nitrous air=nitric oxid (N O), dephlogisticated nitrous air=nitrous oxid (N20), vitriolic acid air=sulphur dioxid, mercurius calcinatus=red oxid of mercury.]



THERE are, I believe, very few maxims in philosophy that have laid firmer hold upon the mind than that air, meaning atmospherical air (free from various foreign matters, which were always supposed to be dissolved, and intermixed with it), is a simple elementary substance, indestructible and unalterable, at least as much so as water is supposed to be. In the course of my inquiries I was, however, soon satisfied that atmospherical air is not an unalterable thing; for that phlogiston with which it becomes loaded from bodies burning in it, and animals breathing it, and various other chemical processes, so far alters and depraves it, as to render it altogether unfit for inflammation, respiration and other purposes to which it is subservient; and I had discovered that agitation in water, the process of vegetation, and probably other natural processes, by taking out the superfluous phlogiston, restore it to its original purity. But I own I had no idea of the possibility of going any farther in this way, and thereby procuring air purer than the best common air. I might, indeed, have naturally imagined that such would be the air that should contain less phlogiston than the air of the atmosphere; but I had no idea that such a composition was possible.

It will be seen in my last publication that, from the experiments which I made on the marine acid air, I was led to conclude that common air consisted of some acid (and I naturally inclined to the acid that I was then operating upon) and phlogiston; because the union of this acid vapor and phlogiston made inflammable air, and inflammable air, by agitation in water, ceases to be inflammable and becomes respirable. And though I could never make it quite so good as common air, I thought it very probable that vegetation, in more favorable circumstances than any in which I could apply it, or some other natural process, might render it more pure.

Upon this, which no person can say was an improbable supposition, was founded my conjecture of volcanoes having given birth to the atmosphere of this planet, supplying it with a permanent air, first inflammable, then deprived of its inflammability by agitation in water, and farther purified by vegetation.

Several of the known phenomena of the nitrous acid might have led me to think that this was more proper for the constitution of the atmosphere than the marine acid; but my thoughts had got into a different train, and nothing but a series of observations, which I shall now distinctly relate, compelled me to adopt another hypothesis, and brought me, in a way of which I had then no idea, to the solution of the great problem, which my reader will perceive I had had in view ever since my discovery that the atmospherical air is alterable, and, therefore, that it is not an elementary substance, but a composition, viz., what this composition is, or what is the thing that we breathe, and how it is to be made from its constituent principles.

At the time of my former publication I was not possessed of a burning lens of any considerable force; and for want of one I could not possibly make many of the experiments that I had projected, and which, in theory, appeared very promising. I had, indeed, a mirror of force sufficient for my purpose. But the nature of this instrument is such that it cannot be applied, with effect, except upon substances that are capable of being suspended or resting on a very slender support. It cannot be directed at all upon any substance in the form of powder, nor hardly upon anything that requires to be put into a vessel of quicksilver; which a appears to me to be the most accurate method of extracting air from a great variety of substances, as was explained in the introduction to this volume. But having afterwards procured a lens of twelve inches diameter and twenty inches focal distance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and factitious, would yield, putting them into the vessels represented in Fig. a, which I filled with quicksilver, and kept inverted in a bason of the same. Mr. Warltire, a good chymist and lecturer in natural philosophy, happening to be at that time in Calne, I explained my views to him. and was furnished by him with many substances, which I could not otherwise have procured.

With this apparatus, after a variety of other experiments, an account of which will be found in its proper place, on the 1st of August, 1774, I endeavored to extract air from mercurius calcinatus per se; and I presently found that, by means of this lens, air was expelled from it very readily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprized me more than I can well express was that a candle burned in this air with a remarkably vigorous flame, very much like that enlarged flame with which a candle burns in nitrous air exposed to iron or liver of sulphur; but as I had got nothing like this remarkable appearance from any kind of air besides this particular modification of nitrous air, and I knew no nitrous acid was used in the preparation of mercurius calcinatus, I was utterly at a loss how to account for it.

In this case, also, though I did not give sufficient attention to the circumstance at that time, the flame of the candle, besides being larger, burned with more splendor and heat than in that species of nitrous air; and a piece of red-hot wood sparkled in it, exactly like paper dipped in a solution of nitre, and it consumed very fast; an experiment which I had never thought of trying with nitrous air.

At the same time that I made the above mentioned experiment, I extracted a quantity of air, with the very same property, from the common red precipitate, which being produced by a solution of mercury in spirit of nitre, made me conclude that this peculiar property, being similar to that of the modification of nitrous air above mentioned, depended upon something being communicated to it by the nitrous acid; and since the mercurius calcinatus is produced by exposing mercury to a certain degree of heat, where common air has access to it, I likewise concluded that this substance had collected something of nitre in that state of heat from the atmosphere.

This, however, appearing to me much more extraordinary than it ought to have done, I entertained some suspicion that the mercurius calcinatus, on which I had made my experiments, being bought at a common apothecary's, might, in fact, be nothing more than red precipitate; though, had I been anything of a practical chymist, I could not have entertained any such suspicion. However, mentioning this suspicion to Mr. Warltire, he furnished me with some that he had kept for a specimen of the preparation, and which, he told me, he could warrant to be genuine. This being treated in the same manner as the former, only by a longer continuance of heat, I extracted much more air from it than from the other.

This experiment might have satisfied any moderate sceptic; but, however, being at Paris in the October following, and knowing that there were several very eminent chymists in that place, I did not omit the opportunity, by means of my friend, Mr. Magellan, to get an ounce of mercurius calcinatus prepared by Mr. Cadet, of the genuineness of which there could not possibly be any suspicion; and at the same time, I frequently mentioned my surprise at the kind of air which I had got from this preparation to Mr. Lavoisier, Mr. le Eoy and several other philosophers, who honored me with their notice in that city; and who, I dare say, cannot fail to recollect the circumstance.

At the same time I had no suspicion that the air which I had got from the mercurius calcinatus was even wholesome, so far was I from knowing what it was that I had really found; taking it for granted that it was nothing more than such kind of air as I had brought nitrous air to be by the processes above mentioned; and in this air I have observed that a candle would burn sometimes quite naturally, and sometimes with a beautiful, enlarged flame, and yet remain perfectly noxious.

At the same time that I had got the air above mentioned from mercurius calcinatus and the red precipitate, I had got the same kind from red lead or minium. In this process that part of the minium on which the focus of the lens had fallen turned yellow. One third of the air in this experiment was readily absorbed by water; but, in the remainder, a candle burned very strongly and with a crackling noise.

That fixed air is contained in red lead I had observed before, for I had expelled it by the heat of a candle, and had found it to be very pure. (Vol. I., p. 192.) I imagine it requires more heat than I then used to expel any of the other kinds of air.

This experiment with red lead confirmed me more in my suspicion that the mercurius calcinatus must get the property of yielding this kind of air from the atmosphere, the process by which that preparation and this of red lead is made being similar. As I never make the least secret of anything that I observe, I mentioned this experiment also, as well as those with the mercurius calcinatus and the red precipitate, to all my philosophical acquaintances at Paris and elsewhere, having no idea at that time to what these remarkable facts would lead.

Presently, after my return from abroad, I went to work upon the mercurius calcinatus which I had procured from Mr. Cadet, and, with a very moderate degree of heat, I got from about one fourth of an ounce of it, an ounce-measure of air, which I observed to be not readily imbibed, either by the substance itself from which it had been expelled (for I suffered them to continue a long time together before I transferred the air to any other place) or by water, in which I suffered this air to stand a considerable time before* I made any experiment upon it.

In this air, as I had expected, a candle burned with a vivid flame; but what I observed new at this time (Nov. 19), and which surprized me no less than the fact I had discovered before, was that whereas a few moments' agitation in water will deprive the modified nitrous air of its property of admitting a candle to burn in it; yet, after more than ten times as much agitation as would be sufficient to produce this alteration in the nitrous air, no sensible change was produced in this. A candle still burned in it with a strong flame, and it did not in the least diminish common air, which I have observed that nitrous air, in this state, in some measure does.

But I was much more surprized when, after two days, in which this air had continued in contact with water (by which it was diminished about one twentieth of its bulk) I agitated it violently in water about five minutes and found that a candle still burned in it as well as in common air. The same degree of agitation would have made phlogisticated nitrous air fit for respiration indeed, but it would certainly have extinguished a candle.

These facts fully convinced me that there must be a very material difference between the constitution of the air from mercurius calcinatus and that of phlogisticated nitrous air, notwithstanding their resemblance in some particulars. But though I did not doubt that the air from mercurius calcinatus was fit for respiration after being agitated in water, as every kind of air without exception on which I had tried the experiment had been, I still did not suspect that it was respirable in the first instance; so far was I from having any idea of this air being what it really was, much superior in this respect to the air of the atmosphere.

In this ignorance of the real nature of this kind of air, I continued from this time (November) to the 1st of March following; having, in the meantime, been intent upon my experiments on the vitriolic acid air, above recited, and the various modifications of air produced by spirit of nitre, an account of which will follow. But in the course of this month I not only ascertained the nature of this kind of air, though very gradually, but was led by it to the complete discovery of the constitution of the air we breathe.

Till this 1st of March, 1775, I had so little suspicion of, the air from mercurius calcinatus, etc., being wholesome that I had not even thought of applying to it the test of nitrous air; but thinking (as my reader must imagine I frequently must have done) on the candle burning in it after long agitation in water, it occurred to me at last to make the experiment; and putting one measure of nitrous air to two measures of this air, I found not only that it was diminished, but that it was diminished quite as much as common air, and that the redness of the mixture was likewise equal to that of a similar mixture of nitrous and common air.

After this I had no doubt but that the air from mercurius calcinatus was fit for respiration, and that it Lad all the other properties of genuine common air. But I did not take notice of what I might have observed, if I had not been so fully possessed by the notion of there being no air better than common air. that the redness was really deeper, and the diminution something greater than common air would have admitted.

Moreover, this advance in the way of truth, in reality, threw me back into error, making me give up the hypothesis I had first formed, viz., that the mercurius calcinatus had extracted spirit of nitre from the air; for I now concluded that all the constituent parts of the air were equally and in their proper proportion imbibed in the preparation of this substance, and also in the process of making red lead. For at the same time that I made the above mentioned experiment on the air from mercurius calcinatus, 1 likewise observed that the air which I had extracted from red lead, after the fixed air was washed out of it. was of the same nature, being diminished by nitrous air like common air: but, at the same time, I was puzzled to find that air from the red precipitate was diminished in the same manner, though the process for making this substance is quite different from that of making the two others. But to this circumstance 1 happened not to give much attention.

I wish my reader be not quite tired with the frequent repetition of the word surprize, and others of similar import; but I must go on in that 6tyle a little longer. For the next day I was more surprized than ever I had been before with finding that after the above mentioned mixture of nitrous air and the air from mercurius calcinatus had stood all night (in which time the whole diminution must have taken place, and, consequently, had it been common air it must have been made perfectly noxious and entirely unfit for respiration or inflammation) a candle burned in it and even better than in common air.

I cannot at this distance of time recollect what it was that I had in view in making this experiment; but I know I had no expectation of the real issue of it. Having acquired a considerable degree of readiness in making experiments of this kind, a very slight and evanescent motive would be sufficient to induce me to do it. If, however, I had not happened, for some other purpose, to have had a lighted candle before me I should probably never have made the trial, and the whole train of my future experiments relating to this kind of air might have been prevented.

Still, however, having no conception of the real cause of this phenomenon, I considered it as something very extraordinary; but as a property that was peculiar to air that was extracted from these substances and adventitious; and I always spoke of the air to my acquaintance as being substantially the same thing with common air. I particularly remember my telling Dr. Price that I was myself perfectly satisfied of its being common air, as it appeared to be so by the test of nitrous air; though, for the satisfaction of others, I wanted a mouse to make the proof quite complete.

On the 8th of this month I procured a mouse and put it into a glass vessel containing two ounce-measures of the air from mereurhts calcinatus. Had it been common air a full-grown mouse, as this was, would have lived in it about a quarter of an hour. In this air, however, my mouse lived a full half hour, and though it was taken out seemingly dead, it appeared to have been only exceedingly chilled; for, upon being held to the fire, it presently revived and appeared not to have received any harm from the experiment.

By this I was confirmed in my conclusion that the air extracted from mercurius calcinatus, etc., was at least as good as common air; but I did not certainly conclude that it was any better; because, though one mouse would live only a quarter of an hour in a given quantity of air, I knew it was not impossible but that another mouse might have lived in it half an hour, so little accuracy is there in this method of ascertaining the goodness of air: and. indeed, I have never had recourse to it for my own satisfaction since the discovery of that most ready, accurate and elegant test that nitrous air furnishes. But in this case I had a view to publishing the most generally-satisfactory account of my experiments that the nature of the thing would admit of.

This experiment with the mouse, when I had reflected upon it some time, gave me so much suspicion that the air into which I had put it was better than common air, that I was induced, the day after, to apply the test of nitrous air to a small part of that very quantity of air which the mouse had breathed so long; so that, had it been common air, I was satisfied it must have been very nearly, if not altogether, as noxious as possible, so as not to be affected by nitrous air; when, to my surprize again, I found that though it had been breathed so long it was still better than common air. For after mixing it with nitrous air, in the usual proportion of two to one, it was diminished in the proportion of 41/2 to 31/2; that is, the nitrous air had made it two ninths less than before, and this in a very short space of time: whereas I had never found that in the longest time any common air was reduced more than one fifth of its bulk by any proportion of nitrous air, nor more than one fourth by any phlogistic process whatever. Thinking of this extraordinary fact upon my pillow, the next morning I put another measure of nitrous air to the same mixture, and, to my utter astonishment, found that it was farther diminished to almost one half of its original quantity. I then put a third measure to it; hut this did not diminish it any farther; but, however, left it one measure less than it was even after the mouse had been taken out of it.

Being now fully satisfied that this air, even after the mouse had breathed it half an hour, was much better than common air, and having a quantity of it still left sufficient for the experiment, viz., an ounce measure and a half, I put the mouse into it; when I observed that it seemed to feel no shock upon being put into it, evident signs of which would have been visible if the air had not been very wholesome; but that it remained perfectly at its ease another full half hour, when I took it out quite lively and vigorous. Measuring the air the next day I found it to be reduced from 11/2 to 2-3 of an ounce measure. And after this, if I remember well (for in my register of the day I only find it noted that it was considerably diminished by nitrous air) it was nearly as good as common air. It was evident, indeed, from the mouse having been taken out quite vigorous, that the air could not have been rendered very noxious.

For my farther satisfaction I procured another mouse, and putting it into less than two ounce-measures of air extracted from mercurius calcinatus and air from red precipitate (which, having found them to be of the same quality, I had mixed together) it lived three quarters of an hour. But not having had the precaution to set the vessel in a warm place, I suspect that the mouse died of cold. However, as it had lived three times as long as it could probably have lived in the same quantity of common air and I did not expect much accuracy from this kind of test, I did not think it necessary to make any more experiments with mice.

Being now fully satisfied of the superior goodness of this kind of air, 1 proceeded to measure that degree of purity with as much accuracy as I could, by the test of nitrous air; and I began with putting one measure of nitrous air to two measures of this air, as if I had been examining common air, and now I observed that the diminution was evidently greater than common air would have suffered by the same treatment. A second measure of nitrous air reduced it to two thirds of its original quantity, and a third measure to one half. Suspecting that the diminution could not proceed much farther, I then added only half a measure of nitrous air, by which it was diminished still more; but not much, and another half measure made it more than half of its original quantity; so that, in this case, two measures of this air took more than two measures of nitrous air and yet remained less than half of what it was. Five measures brought it pretty exactly to its original dimensions.

At the same time air from the red precipitate was diminished in the same proportion as that from mercurius catcinatus, five measures of nitrous air being received by two measures of this without any increase of dimensions. Now as common air takes about one half of its bulk of nitrous air before it begins to receive any addition to its dimensions from more nitrous air, and this air took more than four half measures before it ceased to be diminished by more nitrous air, and even five half measures made no addition to its original dimensions, I conclude that it was between four and five times as good as common air. It will be seen that I have since procured air better than this, even between five and six times as good as the best common air that I have ever met with.



I TOOK a small retort with a long narrow neck, which I bent over a lamp so that the end of the neck could be held under a bell-jar full of water standing in a vessel of water. Into the retort I put two ounces of slightly fuming acid of nitre, the weight of which was to that of distilled water in the proportion of 131,607 to 100,000. I added two ounces one dram of mercury and heated it slightly to hasten the solution.

As the acid was very strong, the effervescence was lively and the decomposition very rapid. I received the air which was liberated in different bell-jars in order to be able to tell the differences which might be found between the air at the beginning and at the end of effervescence, supposing there should be such. When the effervescence had stopped and all the mercury had dissolved, I continued to heat the material in the same apparatus. Soon boiling appeared in place of the effervescence, aud while the boiling went on air was produced in almost as great abundance as before. I continued this until all the fluid had passed out, either by distillation or as elastic vapors of air, and nothing was left in my retort save a white salt of mercury, in a pasty form, dry rather than wet, which began to grow yellow on its surface. The quantity of air obtained up to this point was about 190 cubic inches; that is to say, about four quarts. All this air was of a uniform sort and was nowise different from what M. Priestley has called nitrous air.

On continuing the experiment, I noticed that from the mercury salt there arose red fumes like those of the acid of nitre; but this phenomenon did not last long and soon the air in the empty part of the retort regained its transparence. These red fumes are due to portions of nitrous air and of air purer than ordinary, which are freed from the mercury salt and which combine and form the acid of nitre. The force of this explanation will he fully felt only after the entire memoir has been read. Having put to one side the air which had been given off during the period of the red fumes, I found ten to twelve inches of air very different from what had been given off up till then, and apparently differing from ordinary air only in that lights burned -lightly better in it. At the same time the mercury salt had turned to a fine red precipitate, and, keeping it at a moderate heat, I obtained at the end of seven hours 224 cubic inches of air much purer than ordinary air, in which a light burned with a much brighter, larger and brilliant or more active flame. This air, from all its characteristics, I could riot but recognize as the same that I had extracted from calx of mercury, known as mercury precipitatum per se; the same that M. Priestley extracted from a number of substances by treating them with spirits of nitre. In proportion as this air had been freed, the mercury had been reduced, and I found again, within a few grains, the two ounces one dram of mercury which I had dissolved. The slight loss may have been due to a little yellow and red sublimate which clung to the upper part of the retort.

The mercury came out of this experiment as it went in, that is. without change in its quality or to any noticeable extent in its weight. So it is evident that the 426 cubic inches of air which I had obtained could have been produced only by the decomposition of the acid of nitre. I was then right in concluding from these facts that two ounces of acid of nitre are composed, first, of 190 cubic inches of nitrous air; second, of 12 cubic inches of ordinary air; third, of 224 cubic inches of air better than ordinary air; fourth, of phlegm; but as it was proved from M. Priestley's experiments, that the small amount of common air which I had obtained could be nothing save air better than common air, the superior quality of which had been altered by mixture with nitrous air in the transition or passing from one to the other, I can determine the amount of these two airs before their mixture and suppose that the 12 cubic inches of common air which I got were due to a mixture of 30 cubic inches of nitrous air and 14 cubic inches of air better than ordinary air.

After thus determining these quantities, we get as the product of two ounces of acid of nitre:

Nitrous air 226 cubic inches.
Purest air 238 ""
Total 464

[Lavoisier here uses the estimated weight of the gases found to decide the composition by weight of nitric acid.]

Such then is a way to decompose the acid of nitre and demonstrate the existence in it of a pure air and (if I may he allowed to use this expression) more an air than ordinary air. But the complement of the proof was, after having decomposed the acid, to succeed in re-compounding it out of the same materials, and that is what 1 have done.

[Lavoisier here inserts some preliminary remarks about the nature of nitrous air, and then describes his experiment as follows:]

I filled with water a tube which was closed at one end and which was marked off along its length by equal divisions of volume. I inserted this tube, thus filled with water, in another vessel, likewise filled with water; I let into it seven and one-third parts of nitrous air and mixed with this at the same time four parts of air purer than ordinary air, which I had measured out in another separate tube.[3] At the moment of mixture, the eleven and a third parts of air occupied 12 to 13 measures, but, a moment later, the two airs mingled and combined, very red vapors of spirits of fuming nitre were formed, which were at once condensed by the water, and in a few seconds the eleven and a third parts of air were reduced to about a third of a measure; that is to say, to about the thirty-fourth part of their original volume.

The water contained in the tube was sensibly acid at the end of this experiment, or, rather, it was a weak acid of nitre; when I treated it with alkali I got from it by evaporation real nitre. . . . After having shown that one can separate and combine again the principles of the acid of nitre, it remains for me to show that the same can be done with materials not all taken from the acid of nitre. Instead of the purest air, or that drawn from the red precipitate of mercury, one may use the air of the atmosphere; but much more of it will have to be used, and instead of the four parts of pure air which are sufficient to saturate seven and one third parts of nitrous air, one will have to use nearly sixteen of common air; all the nitrous air is, in this experiment, as in the preceding one, destroyed or rather condensed; but this is not the case with common air; not more than a fifth or a fourth of it is absorbed, and what remains is no longer able to support the flame of a candle or to support respiration in animals. It seems proved by this that the air which we breathe contains only a fourth part of real air; that this real air is in our atmosphere mixed with three or four parts of a harmful air, a sort of choke-damp, which would cause the death of the majority of animals if it were present in a little greater quantity. The injurious effects on the air of vapor of charcoal and of a large number of other emanations prove how near this fluid is to the point beyond which it would be fatal to animals. I hope to soon be in a position to discuss this idea and to place before the Academy the experiments on which it is based.

It results from the experiments contained in this memoir that when mercury is dissolved in nitric acid, this metallic substance acquires the pure air contained in the nitric acid and constituting it an acid. On the one hand this metal, when combined with the purest air, is reduced to a calx; on the other the acid deprived of this same air expands and forms nitrous air, and the proof that such are the facts in this experiment is that if after having thus separated the two airs which enter into the composition of the acid of nitre, you combine them anew, you make pure acid of nitre such as you had before, with the single difference that it fumes.

The acid of nitre, drawn from saltpetre by clay, is consequently nothing but nitrous air combined with nearly an equal volume of the purest part of the air and with a fairly large amount of water; nitrous air, on the contrary, is the acid of nitre deprived of air and of water. People will no doubt ask here if the phlogiston of the metal does not play some part in this process. Without daring to decide a question of so great importance, I will reply that since the mercury comes out of this experiment just as it went in, there are no signs that it has lost or gained any phlogiston, unless we claim that the phlogiston which brought about the reduction of the metal passed through the vessels. But that is to admit of a particular sort of phlogiston, different from that of Stahl and his school; it is to return to the theory of fire as a principle, to fire as an element of bodies, a theory much older than Stahl's and very different from it.

I will end this memoir as I began it, by thanking M. Priestley, to whom the greater part of whatever interest it possesses is due; but the love of truth and the progress of knowledge, towards which all our efforts should be directed, oblige me at the same time to correct a mistake which he has made, which it would be dangerous to leave unchallenged. This rightly famous physicist, who had discovered that when he combined the acid of nitre with any earth, he invariably obtained ordinary air or air better than ordinary air, believed that he could thence draw the conclusion that the air of the atmosphere is a compound of acid of nitre and of earth. This bold conception is quite overthrown by the experiments contained in this memoir. It is clear that it is not air that is composed of acid of nitre, as M. Priestley claims; but, on the contrary, it is the acid of nitre that is composed of air; and this single remark gives the key to a large number of experiments contained in Sections III., IV. and V. of M. Priestley's second volume.



WHEN the chemists of olden times had reduced a body to oil, salt, earth and water, they believed that they had reached the limits of chemical analysis, and consequently they gave to salt and to oil the names of 'principles of bodies.'

In proportion as the art made progress, the chemists who succeeded them became aware that substances which had been held to be primary could be decomposed, and they recognized in succession that all the neutral salts, for instance, were formed by the union of two substances, an acid of some sort and a salt, earth or metal.

Hence arose the entire theory of neutral salts which has held the attention of chemists for over a century, and which is to-day so near perfection that we may regard it as the surest and most complete part of chemistry.

Chemical science has been handed down to us in this condition, and it is our business to do with the constituents of the neutral salts what the chemists who went before us did with the neutral salts themselves, to attack the acids and bases and to carry chemical analysis along this line a step beyond its present limits.

I have already imparted to the Academy my first efforts in this field. I have in earlier memoirs demonstrated to you as far as it is possible to demonstrate in physics and chemistry that the purest air, that to which M. Priestley has given the name of 'dephlogisticated air,' enters as a constituent part into the composition of several acids, notably of phosphoric, vitriolic and nitric acids.

More numerous experiments put me in a position to-day to draw general conclusions from these results and to assert that the purest air, the air most suitable for respiration, is the principle which causes acidity; that this principle is common to all acids, and that in addition one or more other principles enter into the composition of each acid, differentiating it and making it one sort of acid rather than another.

In consequence of these facts, which I already regard as very firmly established, I shall henceforth call dephlogisticated air or air most suitable for respiration, when it is in a state of combination or fixity, by the name of 'the acidifying principle,' or, if one prefers the same meaning in a word from the Greek, 'the principle Oxygine.' This nomenclature will save periphrases, will make my statements more exact, and will avoid the ambiguities I would be likely to fall into constantly if I used the word 'air.'

Without repeating details which I have given elsewhere, I will recall herein a few words, adopting this new language:

1. That the acidifying principle or oxygen, when combined with the substance of tire, heat and light, forms the purest air, that which M, Priestley has called dephlogisticated air; it is true that this first proposition is not strictly proved and perhaps is not susceptible of strict proof; so 1 have proposed it only as an idea that I regard as very probable, and in that respect it must not be confused with the propositions which are to follow, which are based on rigorous experiments and proofs;
2. That this same acidifying principle or oxygen, combined with carbon or substances containing carbon, forms the acid of chalk (carbonic acid) or fixed air:
3. That with sulphur it forms vitriolic acid;
4. That with nitrous air it forms nitric acid;
5. That with Kunckel's phosphorus it forms phosphoric acid;
6. That with metallic substances in general it forms metallic calces, with the exception of the cases of which I shall speak in this or a following memoir.

Such is very nearly our present general knowledge of the combinations of oxygen with the different substances in nature, and it is not hard to see that there remains a vast field to explore; that there is a part of chemistry absolutely new and until now unknown, which will be completely investigated only when we shall have succeeded in determining the degree of affinity of this principle with all the substances with which it can combine, and in discovering the different sorts of compounds which result.

All chemists know that the simpler the substances are with which you are working, the nearer you come to reducing substances to their elementary molecules, the more difficult become the means of decomposing and recomposing the substances; we may suppose, therefore, that the analysis and synthesis of acids must present much greater difficulties than does the analysis of the neutral salts into the composition of which they enter. I hope, however, to be able in what follows to show that there is no acid, unless, perhaps, it be that of sea salt, which w T e cannot analyze and put together again and from which we cannot at will abstract the acidifying principle.

This kind of work demands a great variety of means, and the procedures necessary to success in effecting combination vary according to the different substances with which one is working. In some cases w T e must have recourse to combustion, either in atmospheric air or pure air. Such is the case with sulphur, phosphorus and carbon; these substances during combustion absorb the acidifying principle or oxygen, and by the addition of this principle become vitriolic, phosphoric and carbonic acid or fixed air.

In the case of other substances mere exposure to the air, aided by a moderate degree of heat, suffices to bring about the combination, and this is what happens to all vegetable substances capable of passing on to acid fermentation. In the greater number of cases one has to resort to the science of affinities and to employ the acidifying principle already united in another compound.

The example which I am going to give to-day is of this last sort, and I shall take it from an experiment, well known for several years, following the memoirs of M. Bergman. It is the formation of the acid of sugar. This acid, in accordance with the experiments which I am going to recount, seems to me to be nothing else than sugar combined with the acidifying principle or oxygen, and I propose to show in order in different memoirs that we can combine this same principle with the substance composing animals' horns, with silk, with animal lymph, with wax, with essential oils, with extracted oils, manna, starch, arsenic, iron and probably with a great many other substances of the three kingdoms. "We can thus turn all these substances into genuine acids.

Before entering on the material to be presented, I beg the Academy to recall that the acid of nitre, as shown by the experiments which I have before described, and which I have repeated in your presence, is the result of the union of nitrous air with the purest air or acidifying principle; that the proportion of these two principles varies in the different kinds of acid of nitre, the one which gives off fumes, for instance, containing a superabundance of nitrous air.

  1. From 'Experiments and Observations on Different Kinds of Air.' London, 1775.
  2. Read before the Paris Academy of Science on April 20, 1776. Translated for The Popular Science Monthly from the 'Comptes Rendus' for the meeting.
  3. I pass over the tentative efforts by which I came to know the exact proportion.
  4. Read before the Paris Academy of Sciences on September 5, 1777. Translated for The Popular Science Monthly from the 'Comptes Rendus' for the meeting.