CARBON AND THE HYDROCARBON- 357 



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, CH 3 CH 2 CH 2 CH 3 ; 

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



PITT 



CH. 2 , and there it will consist of CH 3 CH QJJ S - It may be also re- 

 garded 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 part of hydrogen is exchanged for methyl 

 CH 2 CHCH 3 , and trimethylene is ethane, CH 3 CH 3 , only with the 

 exchange of methylene for two hydrogen atoms from two methyl 



pTT 



groups that is, pjj 2 CH 2 , t4 where the methyl introduced is united 



to both the atoms of carbon existing 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 carbons, 

 and, on the other, the different connection between the separate atoms 

 of carbon. In the first case they may be said to be chained together 

 (more generally to form ' an open chain '), and in the second case, to be 

 locked up (to form a ' closed chain ' or ' ring '). Here, also, it is easily 

 understood that on increasing the quantity of carbon atoms the num- 

 ber of conjectured and existing isomerides will greatly increase. If, 

 also, 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 parts of the hydrogen be exchanged for X 2 , one 



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

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

 as there were diversities in the atoms of hydrogen in CH 4 . The scope of this work does 

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

 chemistry. 



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

 as an explanation of the structure and isomerism of the derivatives of benzene, C^Hg, 

 forming aromatic compounds (Note 26). It is now incontestable, although the ques- 

 tion of isomerism between the benzene derivatives cannot be considered as finally 

 solved, that this closed connection exists, and gives a peculiar character to these com- 

 pounds. 



