PARAFFIN. In the course of his classical investiga-tion on the tar produced in the dry distillation of wood, Reichenbach in 1830 discovered in it, amongst many other things, a colourless wax-like solid which he called paraffin (parum aginis) because he found it to be endowed with an extraordinary indifference towards all reagents. A few years later he isolated from the same material a liquid oil chemically similar to paraffin, to which he gave the name of eupion (eindow, very fat). For many years both these bodies were known only as chemical curiosities, and even scientific men looked upon them as things entirely sui generis; this wa.s natural enough as far as paraffin is concerned, but it is rather singular that it took so long before it was realized that eupion or something very much like it forms the body of PETROLEUM (q.v.), which had been known, since the time of Herodotus at least, to well up abundantly from the bowels of the earth in certain places. Though extensively known, it was used only as an external medicinal agent, until the late Air James Young conceived the idea of industrially working a com-paratively scanty oil-spring in Derbyshire, and subse-quently found that an oil similar to petroleum is obtained by the dry distillation of cannel coal and similar materials at low temperatures. This discovery developed into a grand industry, which may be said to have led to the utilization of those immense natural stores of petroleum in America. Scientific chemists naturally directed their attention to the products of these new industries, and it was soon ascertained that solid paraffin and eupion, as well as natural and artificial petroleum, are substantially more or less impure mixtures of saturated hydrocarbons ; and so it comes that, on the proposal of H. Watts, the word paraffin in scientific chemistry has been adopted as a generic term for this class of compounds of carbon and hydrogen.

When the electric light is generated within an atmo-sphere of hydrogen, then, at the immense temperature of the electric arc, part of the carbon of the charcoal terminals unites with the hydrogen into acetylene gas, 0,112. Apart from this isolated fact, which was discovered by Berthelot in 1862, it might be said that the two elements are not capable of uniting directly, although an innumerable the majority present themselves as liquids ; not a few are solids. But the solids are fusible ; and all liquid or liquefied hydrocarbons, at a high enough temperature, volatilize, as a rule without decomposition. To the latter circumstance to a great extent we owe our precise knowledge of their chemical constitution.

In all the numerous series of hydrocarbons the percentages of carbon vary front 75 (in marsh gas) to 94.7 (in chrysene). Within this narrow range of some 20 per cent. several dozens of elementary compositions have to he accommodated ; and many of these, to be represented in formulae C211, with an adequate degree of precision, require formulw in which the coefficients x and y are so large that, by means of integers less than these, any fancy composition (within our liinits) may be expressed with a degree of exactitude which is quite on a par with the analyses. But these hydrocarbons, in general, can be volatilized into gases, and in regard to these Avogadro's law tells us that quantities proportional to the mole-cular weights (i.e., the weights represented by the true chemical formulze) occupy the same volume. Hence, to find the true value, M=C2H„, of the formula as a whole, wc need only determine the vapour density, and from it calculate the weight of the respective hydrocarbon which, as a gas at r and P millimetres pressure, occupies the same volume as, for instance, H20 parts of steam. This is M. The elementary analysis enables us to calculate the weight x x C of carbon contained in 31 parts, and the analysis must be very poor to leave us in doubt as to whether it is for instance 6 x 12 parts of carbon or 7 x 12 parts that we have to deal with. The reader will now understand how it has been possible to ascer-tain the elementary composition of all pure hydrocarbons with a degree of precision which goes beyond that of the analysis, and to prove what analysis could never have done by itself, namely, that there are numerous groups of hydrocarbons which have absolutely identical elementaly compositions, - cases of isomerism, as they are called. We speak of " isomerism in the narrower sense " when the atomic fornaul are identical (there are, for instance, two hydrides of butyl, C4H,o), while we speak of " polymeric " bodies when the several formulT are integer multiples of the same primi-tive group (e.g., ethylene, 2 x CH2, and butylene, x CH2, are polymers to one another).

The following table gives au idea of the several classes of hydro-carbons which for us come rnore particularly into consideration.

Benzol. Marsh gas.

But similarly two dehydrogenated henzols, C2112, can unite into oue double ring of diphenyl : 2C611, - 211 - (C2142)(C2H5); and two benzol rings may unite more firmly in such a manner that two carbon atoms of the one ring do service for the two rings, and a double ring is formed firmly united by these two connnon carbons, the four hydrogens of the original two benzols being away. This gives naphthalene : - C61-16+ C„1 I„ - 2C - 4H =C,0118.

Benzol. Naphthalene.

In a similar manlier throe benzols may unite into oue with-racene -