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 MC n 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, C 6 H 5 Br, and ethyl bromide, C 2 H 5 Br, be heated with metallic sodium, the sodium combines with the bromine of both compounds, forming sodium bromide, NaBr. From the first combination the group C 6 H 5 remains, and from the second C 2 H 5 . 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 C 6 H 5 .C 2 H 5 or C 8 H 10 (ethylbenzene). Hydrocarbons are also produced by the breaking up of more complex organic or