Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/256

238 PARAFFIN In a similar manner three benzols may unite into one anth racene : C 6 H 6 + C 6 H 6 + C 6 H 8 - 40 - 8H - C J4 H 10 . Benzol. Anthracene. Generally speaking, a hydrocarbon is the more volatile the less the number of carbon atoms and the greater the number of hydrogen atoms in the molecule. Thus, in the series of "paraffins," CH 4 (marsh gas) and C 2 H 6 (ethane) are gases, C ;) H S (propane) and C 4 H 10 (butane) are very volatile liquids, and C 5 H 12 , fec., are liquids, with higher and higher boiling points as we ascend the series. From a certain value of n upwards we find ourselves amongst the paraffins proper, which are solids, more or less easily fusible, but not, in general, volatile without decomposition. Benzol, C H 6 , and its neighbouring homologues are volatile liquids. Naphthalene and anthracene are crystalline solids, fusible at 79 2 and 180 C., and boiling at 217 and above 300 C. respectively without decomposition. All hydrocarbons agree in this, that they are practically insoluble in water, but more or less readily soluble (in general) in alcohol and in ether. They are all combustible ; the more readily volatile ones are inflammable. Any complete combustion, of course, leads to the formation of only carbonic acid and water, with evolution of a large amount of heat ; but the mechanism of the process is more or less complex. Naphthalene and anthracene remain un- decomposed at a red heat; only at the very high tempera ture of their flames, and by the co-operation of the oxygen of the air, they are decomposed with large elimination of charcoal; a similar, though less, stability is exhibited by the benzols. The paraffins, on the other hand, are relatively unstable. Marsh gas, it is true, stands a red heat ; but, to pass to the other end of the series, the paraffins proper, and also the higher liquid paraffins to some extent, even when being distilled, and especially when distilled "under pressure," i.e., at higher temperatures than their natural boiling points, break up into olefines and lower paraffins (Thorpe and John Young). Similar changes take place when the vapours of paraffins are passed through red-hot tubes; only the products formed then suffer deeper-going decomposition with formation of hydrogen, marsh-gas, acetylene, ethylene, and charcoal, and, last not least, benzols and naphthalene. To this latter fact the paraffins owe their pre-eminent fitness as illuminating agents. When organoid minerals, such as cannel coal, shale, &c., aVe subjected to dry distillation, all the several classes of hydrocarbons are in general produced at the same time; but, from what we have said it will be understood that, even with the same material, the quantitative composition of the complex vapour which comes out of the retort depends on the way in which the distillation is being conducted. If we operate at the lowest practicable temperature, comparatively little gas is produced, and in the condensible part of the vapour the paraffins pre dominate largely ; at a bright red heat, such as is used in making coal gas, and especially if the vapours have to pass along red-hot surfaces before they get into the condenser pipes, more gas is produced, and the place of the liquid paraffins is taken by benzols. These latter, however, are always accompanied by naphthalene, often also by anthra cene, and invariably by certain ternary benzol-derivatives, namely, by "phenols," feebly acid bodies containing hydroxyl groups, OH s, where the corresponding hydro carbon bore plain hydrogens (ordinary phenol, C 6 H 5 (OH), derived from benzol, C C H 5 H, is a representative example), and, secondly, basic compounds of carbon, hydrogen, and nitrogen. Of the latter aniline and picoline both C H r N, but widely different in their properties may be quoted as examples. The gas produced in this case through the presence in it of the vapour of higher hydrides, but especially of acetylene, C 2 H 2 , and benzol is highly luminous. Supposing now, as a third instance, the distillation to be conducted at a white heat, and so that the primary vapour has to wind its way through a spiral pipe kept at a bright red heat, the pro portion of gas increases largely, and there is an increased yield of retort charcoal; but the liquid hydrocarbons of all classes almost vanish; the gas consists mainly of hydrogen, marsh gas, carbonic oxide, and carbonic acid, and gives little light when kindled. The aim of the paraffin oil manufacturer is to produce the best possible approximation to a mixture of paraffins, wherefore he conducts his distillation at the lowest work ing temperature. Of course his paraffin mixture contains more or less of the other classes of bodies referred to, whose removal, however, offers no great difficulty. In the laboratory we should commence by shaking the crude oil with caustic alkali ley, which withdraws the phenols and other acid bodies, as part of a lower layer, the upper being purified oil. By shaking the latter with dilute sulphuric acid the bases are removed as a solution of their sulphates, and a still purer oil results. Application of con centrated sulphuric acid to the latter removes part at least of the benzols and olefines as sulpho-acids, and also of the phenols and all the bases, should the two preceding operations have been omitted. But the most thorough mode of getting quit of the benzols and their derivatives is after having exhausted the milder agents to shake the oil with first aqueous and then stronger and stronger nitric acid, which reagent converts the benzol-bodies into nitro- products, soluble in the acid, or removable, after separation of the acid layer, by aqueous alkali. By all these tortures the paraffins being what the name implies are not much affected, so that what ultimately survives all belongs to their family. The separation of the individual paraffins from one another is a very difficult problem which has not yet found a satisfactory solution. What we know of in dividual paraffins is derived chiefly from the investigation of decompositions of pure chemical substances leading to the formation of that one paraffin principally if not solely. To split up a mixture of paraffins approximately the only known method is fractional distillation (see DISTILLATION, vol. vii. p. 260), preferably by means of an apparatus so constructed that the vapour, before reaching the con denser, ascends through an intermediate inverted con denser or still-head, and there suffers partial condensation at some suitable temperature (enforced in the most perfect form of the apparatus by an oil-bath surrounding the still- head). In this latter case, singularly not as a matter of course by any means what goes over boils very nearly at the temperature of the still-head. This particular form of the method therefore lends itself chiefly for the final purification of an unitary substance of known boiling point already purified by preceding distillations. With mixtures of unknown composition the process is very tedious, and may assume something like this form. We distil tho cubstance (slowly and with ample chance of partial condensation) and collect as separate fractions what came over at, for instance, 100 to 105, 105 to 110, 110 to 115, <fec., as I., II., III., IV., &c. Each of these when redistilled yields I. and II. and III. and IV., &c., which parts are poured into the respective receptacles, and on this principle Ave continue working. If the sub stance happens to be of comparatively simple composition, it usually turns out, after a while, that (say) the two fractions II. and VI. increase while the rest get less and less ; and by working on we may be able to isolate two bodies of the constant boiling points t. 2 and t f> respectively, with formation of " tails " of other boiling points. Unfortunately, even a constant boiling point is no proof of chemical purity ; and, if a constant-boiling substance is a