CARBON AND THE HYDROCARBONS 36? 



(CH) (CH), or acetylene, both the latter being unsaturated hydro- 

 carbons. Thus we have all the possible hydrocarbons with two atoms 

 of carbon in the molecule, C 2 H 6 , ethane, C 2 H 4 , ethylene, and C 2 H 2 , 

 acetylene. But in them, according to the law of substitution, the 

 same forms of substitution may be repeated that is, the methyl, 

 methylene, 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 fresh series of 

 saturated and unsaturated hydrocarbons, 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 

 means of the law of substitution we can foresee not only the limit 

 C rt H 2 , 1+2 , but an unlimited number of unsaturated hydrocarbons, C M H 2 " n , 

 C,,H 2 _2 .... C ;t H 2 (_,), 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 intelligible, but it is also seen that there is a possi- 

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

 polymerise so common to hydrocarbon compounds is already apparent 

 in the first unsaturated series C n H 2tt , because all the terms of this 

 series C 2 H 4 , C 3 H 6 , 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. The differences in the vapour 

 density, boiling points, and melting points, of the quantities 

 entering into reactions, 39 and the methods of preparation 4 . also so 

 clearly tally with the conception of 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. 41 The first boils at 81, the second at 144; 



38 When m = n 1, we have the series C U H 2 . The lowest member is acetylene, C 2 H 2 . 

 These are hydrocarbons containing a minimum amount of hydrogen. 



59 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 2 n. 



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

 C 2 H 5 (OH), and amylene, C 5 H 10 , from amyl alcohol, C 6 H n (OH), or in general CH 2/ ,,from 



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

 its homologues (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 substances 

 CsoHga and CsoHgo differ so slightly in their composition by weight as to be within the 

 limits of experimental error, but their reactions and properties are so distinct that' they 



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