CARBON AND THE HYDROCARBONS 369 



is to be expected, however, that there should be two butanes, C 4 H 10 , 

 and this is actually the case. In one, methyl may be considered 

 as replacing the hydrogen of one of the methyls, <JH 3 CH 2 CH 2 CH 3 ; 



and in the other CH 3 may be considered as substituted for H in 



OFT 

 CH 2 , and there it will consist of CH 3 CH^g 3 . 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. C 2 H 4 and CH 2 CH 2 are, it is evident, 

 identical ; but there ought to be, and are, two hydrocarbons of the 

 composition C 3 H 6 , propylene and trimethylene ; the first is ethylene, 

 CH 2 CH 2 , in which one atom of hydrogen is exchanged for methyl, 

 CH 2 CHCH 3 , and trimethylene is ethane, CH 3 CH 3 , with the substi- 

 tution of methylene for two hydrogen atoms from two methyl groups 



CH 



that is, ,rr 2 CH 2 , 44 where the methylene introduced is united to both 



the atoms of carbon in CH 3 CH 3 . It is evident tliat 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 chain ' 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 exchange of part of the hydrogen for some of 

 the other groups of elements X, Y . . . . occurs, the quantity of 

 possible isomerides still further increases in a considerable degree. 

 For instance, there are even two possible isomerides for the derivatives 

 of ethane, C 2 H 6 : if two atoms of the hydrogen be exchanged for X 2 , 



meana is replaced by X, or else, for instance, if CH 5 X be obtained by the decomposition 

 of more complex compounds, the same product is always obtained. 



This was shown in the year 1860, or thereabout, by many methods, and is the funda- 

 mental conception of the structure of hydrocarbon compounds. If the atoms of hydrogen 

 in methyl were not absolutely identical in value and position (as they are not, for instance, 

 in CH 5 CH 2 CH 5 or CH 3 CH 2 X), then there would be as many different forms of CH 5 X 

 as there were diversities in the atoms of hydrogen in CH^ The scope of this work doe$ 

 not permit of a more detailed account of this matter. It is given in works on organic 

 chemistry. 



44 The union of carbon atoms in closed chains or rings was first suggested by Kekule 

 as an explanation of the structure and isomeriam of the derivatives of benzene, 

 forming aromatic compounds (Note 26). 



