n H 2 n + 2 . These so-called saturated hydrocarbons are incapable of combination. The hydrocarbons CH 6 , C 2 H 8 , C 3 H 10 , &c.... do not exist. Those containing the maximum amount of hydrogen will be represented by CH 4 ( n = 1, 2 n + 2 = 4), C 2 H 6 ( n = 2), C 3 H 8 (n = 3), C 4 H 10 , &c. This may be termed the law of limits . Placing this in juxtaposition with the law of even numbers, it is easy to perceive that the possible hydrocarbons can be ranged in series, the terms of which may be expressed by the general formulæ C n H 2 n +2 , C n H 2 n , C n H 2 n -2 , &c.... Those hydrocarbons which belong to any one of the series expressible by a general formula are said to be homologous 0 with one another. Thus, the hydrocarbons CH 4 , C 2 H 6 , C 3 H 8 , C 4 H 10 , &c.... are members of the limiting (saturated) homologous series C n H 2 n +2 . That is, the difference between the members of the series is CH 2 . Not only the composition but also the properties of the members of a series tend to classification in one group. For instance, the members of the series C n H 2 n +2 are not capable of forming additive compounds, whilst those of the series C n H 2 n are capable of combining with chlorine, sulphuric anhydride, &c.; and the members of the C n H 2 n -6 group, belonging to the coal tar series, are easily nitrated (give nitro-compounds, Chapter VI.), and have other properties in common. The physical properties of the members of a given homologous series vary in some such manner as this; the boiling point generally rises and the internal friction increases as n increases—that is, with an increase in the relative amount of carbon and the atomic weight; the specific gravity also regularly changes as n becomes greater.
Many of the hydrocarbons met with in nature are the products of organisms, and do not belong to the mineral kingdom. A still greater number are produced artificially. These are formed by what is termed the combination of residues. For instance, if a mixture of the vapours of hydrogen sulphide and carbon bisulphide be passed through a tube in which copper is heated, this latter absorbs the sulphur from both the compounds, and the liberated carbon and hydrogen combine to form a hydrocarbon, methane. If carbon be combined with any metal and this compound MCn be treated with an acid HX, then the haloid X will give a salt with the metal and the residual carbon and hydrogen will give a hydrocarbon. Thus cast iron which contains a compound of iron and carbon gives liquid hydrocarbons like naphtha under the action of acids. If a mixture of bromo-benzene, C6H5Br, and ethyl bromide, C2H5Br, be heated with metallic sodium, the sodium combines with the bromine of both compounds, forming sodium bromide, NaBr. From the first combination the group C6H5 remains, and from the second C2H5. Having an odd number of hydrogen atoms, they, in virtue of the law of even numbers, cannot exist alone, and therefore combine together forming the compound C6H5.C2H5 or C8H10 (ethylbenzene). Hydrocarbons are also produced by the breaking up of more complex organic or hydrocarbon compounds, especially by heating—that is, by dry distillation. For instance, gum-benzoin contains an acid called benzoic acid, C7H6O2, the vapours of which, when passed through a heated tube, split up into carbonic anhydride, CO2, and benzene, C6H6. Carbon and hydrogen only unite directly in one ratio of combination—namely, to form acetylene, having the composition C2H2, which, as compared with other hydrocarbons, exhibits a very great stability at a somewhat high temperature.33
There is one substance known among the saturated hydrocarbons composed of 1 atom of carbon and 4 atoms of hydrogen; this is a compound containing the highest percentage of hydrogen (CH 4 contains 25 per cent. of hydrogen), and at the same time it is the only hydrocarbon whose molecule contains but a single atom of carbon. This saturated hydrocarbon, CH 4 , is called marsh gas or methane . If vegetable or animal refuse suffers decomposition in a space where the air has not free access, or no access at all, then the decomposition is accompanied with the formation of marsh gas, and this either at the ordinary temperature, or at a comparatively much higher one. On this account plants , when decomposing under water in marshes , give out this gas. It is well known that if the mud in bogs be stirred up, the act is accompanied with the