COAL AND THE COAL-TAR COLORS. 203 



Sometimes contrary forces are developed simultaueously, and bod- 

 ies are at the same time subjected to an influence which brings them 

 together and to another one which separates them ; the result will 

 depend upon slight differences in the temperature or in the propor- 

 tions of the different bodies present. Benzine and carbonic acid unite 

 to form benzoic acid ; benzoic acid decomposes into carbonic acid and 

 benzine. Styrolene is produced by the union of benzine and olefiant 

 gas, and in decomposing yields benzine and acetylene. Benzine makes 

 its appearance again if anthracene and naphthaline are heated in the 

 presence of hydrogen. Sometimes, between these contrary forces, an 

 equilibrium is established. Thus, acetylene will combine with hydro- 

 gen and form olefiant gas ; but olefiant gas will decompose at the same 

 temperature, giving out its two elements ; while, if the three gases 

 are present and all pure, action will be suspended, for the opposing 

 tendencies will be counterbalanced. 



These are only a few of the examples of the reactions that take 

 place when organic substances are raised to a high temperature. The 

 four simple substances entering into the constitution of organic bodies 

 form among one another more compounds than are furnished by all 

 the minerals. If heating takes place in the open air, combustion en- 

 sues, and all these innumerable substances are oxidized and dissipated 

 in the atmosphere as carbonic acid and aqueous vapor. But, if we 

 work in a medium free from oxygen and all other foreign elements, 

 they react upon one another, and a multitude of bodies are formed or 

 decomposed by the interchange of elements, and the mixture we get 

 when the heat is removed is a mixture of new elements. So, solid and 

 dry coal gives the coal-tar liquids and illuminating gas, which did not 

 exist in it, but were formed under the influence of heat. 



Of what organic substances coal is really composed we know only 

 imperfectly. Chemists have not succeeded in making real analyses 

 of it. We can tell how much of impurities, such as sulphuret of iron, 

 it contains, and how much coal-tar and gas can be got from it ; we 

 may classify a specimen as a rich, a poor, or a bituminous coal, or as 

 one giving a long or a short flame, but we do not separate and deter- 

 mine the chemical elements. 



The analyst has not very many resources at his disposal for sepa- 

 rating an intimate mixture of several bodies. The first means is that 

 of distillation. Different bodies sublime at different temperatures, 

 according to their various degrees of volatility ; each of them, under 

 the same atmospheric pressure, passes from the solid to the liquid 

 state at one temperature, and from the liquid to the gaseous at an- 

 other. These temperatures are called, respectively, the point of fusion 

 and the boiling-point. Fractional distillations are performed in ac- 

 cordance with this principle. When the heat is raised to a certain 

 degree, one class of bodies, at a higher temperature another class of 

 bodies, which had not reached their boiling-point at the former temper- 



