polymerism, that this example will always be the clearest and most conclusive for the illustration of polymerism and molecular weight. Such a case is also met with among other hydrocarbons. Thus benzene, C 6 H 6 , and cinnamene, C 8 H 8 , correspond with the composition of acetylene or to a compound of the composition CH. The first boils at 81°, the second at 144°; the specific gravity of the first is 0·899; that of the second, 0·925, at 0°—that is, here also the boiling point rises with the increase of molecular weight, and so also, as might be expected, does the density.
Cases of isomerism in the restricted sense of the word—that is, when with an identity of composition and of molecular weight, the properties of the substances are different—are very numerous among the hydrocarbons and their derivatives.
Such cases are particularly important for the comprehension of molecular structure and they also, like the polymerides, may be predicted from the above-mentioned conceptions, expressing the principles of the structure of the carbon compounds47 based on the law of substitution.
According to it, for example, it is evident that there can be no isomerism in the cases of the saturated hydrocarbons C2H6 and C3H8, because the former is CH4, in which methyl has taken the place of H, and as all the hydrogen atoms of methane must be supposed to have the same relation to the carbon, it is all the same which of them be subjected to the methyl substitution—the resulting product can only be ethane, CH3CH3;48 the same argument also applies in the case of propane, CH3CH2CH3, where one compound only can be imagined.
It is to be expected, however, that there should be two butanes, C4H10, and this is actually the case. In one, methyl may be considered as replacing the hydrogen of one of the methyls, CH3CH2CH2CH3; and in the other CH3 may be considered as substituted for H in CH3, and there it will consist of CH3CH
CH3 / CH3 .
The latter may also be regarded as methane in which three of hydrogen are exchanged for three of methyl. On going further in the series it is evident that the number of possible isomerides will be still greater, but we have limited ourselves to the simplest examples, showing the possibility and actual existence of isomerides. C2H4 and CH2CH2 are, it is evident, identical; but there ought to be, and are, two hydrocarbons of the composition C3H6, propylene and trimethylene; the first is ethylene, CH2CH2, in which one atom of hydrogen is exchanged for methyl, CH2CHCH3, and trimethylene is ethane, CH3CH3, with the substitution of methylene for two hydrogen atoms from two methyl groups—that is,
CH2
CH 2 / CH 2 , where the methylene introduced is united to both the atoms of carbon in CH 3 CH 3 . It is evident that the cause of isomerism here is, on the one hand, the difference of the amount of hydrogen in union with the particular atoms of carbon, and, on the other, the different connection between the several atoms of carbon. In the first case they may be said to be chained together (more usually to form an ‘open chain’), and in the second case, to be locked together (to form a ‘closed chai’ or ‘ring’). Here also it is easily understood that on increasing the quantity of carbon atoms the number of possible and existing isomerides will greatly increase. If, at the same time, in addition to the substitution of one of the radicles of methane for hydrogen a further