THE SOURCE OP LIGHT, HEAT, COLOUR, ETC. 61 



The essential or spirituous oils are also, for the most part, combinations 

 of these elements, and are chemically distinguished as the hydro-carbon 

 oils. Sugars, starches, oils and gums, as well as the proximate principles 

 of plants in general, are likewise mixtures of hydrogen and carbon, so 

 universally prevalent in nature are these substances ; and it is not, conse- 

 quently, surprising that fossil vegetation should be so largely composed of 

 the same bodies. 



An intimate relationship exists between the fossil and recent hydro- 

 carbons : indeed, so far as those of vegetable origin are concerned, it is 

 doubtful whether the former have undergone any very sensible modifica- 

 tion by the mineralising process to which they have been subjected. If we 

 distil wood, we obtain products almost precisely similar to those procured 

 from coal, — as gas, tar, light and heavy oils, &c. : indeed, from this source, 

 or from the saps of certain recent plants, even a supply of those hydro- 

 carbons of modern discovery, and increasing interest, are forthcoming. 

 Benzole can be obtained from gum benjamin, and other sources. Aniline 

 is largely dispersed in indigo. Naphthaline also, which is the newest base 

 of a beautiful blue colour, is equally found in wood as in coal naphtha. 



It may then be argued, from the evident connection of the fossil and 

 recent bydro-carbons, that the most obvious course would be to procure 

 them, as it were, first-hand ; but experience teaches us that although under 

 favourable conditions, as to population and territory, we may derive light, 

 heat, and colour from existing vegetation, yet that the hidden stores of 

 hydro-carbon are far more generally available, and to England, at least, the 

 only available source of supply. Coal is obviously cheaper than wood ; 

 and the products of coal, whether natural or artificial, are necessarily less 

 costly than any analogous substances obtained by man from recent 

 plants. 



The precise method by which wood is converted into coal is suggested 

 by Thomson to be by a simple deoxidation of the former. " If," he writes, 

 " we subtract from woody fibre C 36 H 22 22 , 3 atoms H, 3 atoms H O, and 

 "9 atoms C 2 , we get C 24 H 13 O — the formula for cannel or splint coal:" 

 or, put into simple language, if certain cpuantities of hydrogen gas, carbonic 

 acid, and pure water be abstracted or evolved from woody matter, the 

 residuum is a certain sort of coal. 



The relationship existing between the fossil and recent hydro-carbons 

 is rendered still further apparent by the property of elasticity which both 

 possess to a limited extent. Certain well-known saps from recent plants 

 found in various parts of the globe are pure hydro-carbons, having great 

 elasticity, — india-rubber, gutta-percha, &c. ; whilst, if we turn to the fossil 

 world, we find a bituminous substance with the following composition — 

 carbon 85 - 5, hydrogen 133, and likewise possessed of elasticity. This 

 substance is elastic bitumen, or mineral caoutchouc, as it is advisedly 

 called ; it will remove the marks of a graphite pencil precisely in the same 

 manner as the vegetable caoutchouc, and, but for its great scarcity, it might 

 possibly be utilised, and help to swell the supply of a material in many 

 ways essential to the age in which we live. 



