354 PRINCIPLES OF CHEMISTRY 



the law of even numbers is seen from the fact that in all the cases of 

 substitution mentioned the hydrogen atoms increase or decrease by 

 an even number ; but, as in CH 2 they are likewise even, it follows that 

 no matter how many substitutions they produce there will always be 

 an even number of hydrogen atoms obtained. Indeed, when H is re- 

 placed by CH 3 there is an increase of CH 2 ; when H 2 is replaced by 

 CH 2 there is no increase of hydrogen ; in the acetylene substitution 

 CEI replaces H 3 , therefore there is an increase of C and a decrease of 

 H 2 ; in the carbon substitution there is a decrease of H 4 . In a similar 

 way the law of limit may be deduced as a corollary of the law of 

 substitution. In fact, the largest possible quantity of hydrogen 

 introduced corresponds with the methyl substitution, but it leads 

 to the introduction of CH 2 ; therefore, no matter how many times this 

 exchange may be effected with CH 4 , and, denoting the number of times 

 by (w-1), the result will always be CH 4 (w-l)(CH 2 ), or C,,H 2n+2 

 as the highest hydrogen compound. TJnsaturated hydrocarbons, con- 

 taining less hydrogen, are evidently only formed when the increase of 

 the new molecule derived from methane proceeds from one of the other 

 forms of substitution. When the methyl substitution alone takes place 

 with methane, CH 4 , it is evident that the saturated hydrocarbon formed 

 is C 2 H 6 or (CH 3 )(CH 3 ). 37 This is called ethane. Directly, by means 

 of the methylene substitution alone, ethylene, C 2 H 4 , or (CH 2 ) (CH 2 ) may 

 be obtained from CH 4 , and by the acetylene substitution, C 2 H 2 or 

 (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 15 ethylene, arid C 2 H 2 , 

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

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



37 Although the methods of formation and the reactions connected with hydrocarbons 

 are not described in this work, because they are dealt with in organic chemistry, yet, in 

 order to clearly show the mechanism of those transformations by which the carbon 

 atoms are built up in the molecules of the carbon compounds, we here give a general 

 example of reactions of this kind. From marsh gas, CH 4 , on the one hand, the substi- 

 tution of chlorine or iodine, CH 5 C1, CH 3 I, for the hydrogen may be effected, and, on the 

 other hand, such metals as sodium may be substituted for the hydrogen, CH 3 Na. Such 

 and other similar products of substitution are exceedingly characteristic of hydrocarbons, 

 and serve as a means of obtaining other more complex substances from given carbon com- 

 pounds. If we place the two above-named products of substitution of marsh gas i metallic 

 and haloid) in mutual contact, the metal combines with the halogen, forming a very stable 

 compound namely, common salt or sodium chloride and the carbon-groups which were 

 in combination with them separate in mutual combination, as shown by (lie equation : 



CH 5 C1 + CH 3 Na = NaCl + C 2 H 6 . 



This is the most simple example of the formation of a complex hydrocarbon from these 

 * radicles. The cause of the reaction must be sought for in the property which chlorine 

 and sodium have of entering into mutual combination. 



