Sometimes contrary forces are developed simultaneously, and bodies 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 proportions 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 hydrogen 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 ensues, 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 determine the chemical elements.
The analyst has not very many resources at his disposal for separating 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 another. These temperatures are called, respectively, the point of fusion and the boiling-point. Fractional distillations are performed in accordance 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-
