CARBON AND THE HYDROCARBONS 355 



inethylene, acetylene, and even carbon substitutions (because C 2 H 6 will 

 still contain hydrogen when C replaces H 4 ) and therefore further sub- 

 stitutions will serve as a source for the production of a set of saturated 

 and unsaturated hydrocarbons, all containing more and more carbon 

 in the molecule and, in the case of the acetylene substitution and 

 carbon substitution, containing less and less hydrogen. Thus by 

 IH> <uis of the law of substitution we can foresee not only the limit 

 C,,H 2 , l+2 , but an unlimited number of unsaturated hydrocarbons, C, ( H 2)1 , 

 C rt H., n _ 2 .... C H H. 2 (,,_ m ), where m varies from to n - I, 38 and 

 where n increases indefinitely. From these facts not only does the 

 existence of a multitude of polymeric hydrocarbons, differing in mole- 

 cular weight, become evident, but it is also seen that there is a possi- 

 bility of cases of isomerism with the same molecular weight. Already 

 in the first unsaturated series C w H 2n that polymerism so common to 

 hydrocarbon compounds is apparent, because all the terms of this 

 series C 2 H 4 , C 3 H r ,, C 4 H 8 .... C 30 H 60 .... have one and the 

 same composition CH 2 , but different molecular weights, as has been 

 already explained in Chapter VII. (page 312). The differences in the 

 vapour density, boiling points, and melting points, of the quanti- 

 ties entering into reactions, 39 and the means of preparation 40 also so 

 clearly tally with the doctrine of polymerism, that this example will 

 always be the clearest and most conclusive for the conception 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. 41 The first boils at 81, the second at 144 ; 



38 When m = n 1, we have the series C,,H._). The lowest member is acetylene, C.H.<>. 

 Amongst the hydrocarbons with C-, there ought to be two of the formula CsHo, one 

 with the structure CH,CCH, and the other CCH.,,C. But one of these is unknown. For 

 C 4 H>, CHCCCH is known the diacetylene of v. Baeyer, an explosive, easily destructible, 

 gaseous substance. Nothing more is known of the series C,,Ho. 



39 For instance, ethylene, C 2 H 4 , combines with Br 2 , HI, H 2 SO 4 , as a whole molecule, 

 as also does amylene, C 5 H 10 , and, in general, C n H 2rt that is, the reacting masses are 

 l>n>i>orti<>n;il to the molecular weight or n, or to the density of the vapours (page 307). 



40 For instance, ethylene is obtained by removing the water from ethyl alcohol, 

 C.,,H 5 (OH), and amylene, C 5 H 10 , from amyl alcohol, Cr,H u (OH), or in general C lt H.yi from 



41 Acetylene and its polymerides have an empirical composition CH, ethylene and 

 its homolognes (and polymerides) CH 2 , ethane CH 3 , methane CH 4 . This series presents 

 a good example of the law of multiple proportions, but such diverse proportions are met 

 with between the number of atoms of the carbon and hydrogen in the hydrocarbons 

 already known that the accuracy of Dalton's law might be doubted. Thus the difference 

 in weight between the substances C 30 H 02 and CsoHgQ is so slight that it is within the in- 

 evitable errors of analysis, but their reactions and properties are so distinct that they 

 can be distinguished beyond a doubt. Without Dalton's law, chemistry could not have 

 brrn brought to its present condition, but it cannot alone express all those shades which 

 -are quite clearly understood and predicted by the law of Avogadro-Gerhardt. 



A A 2 



