866 PEINCIPLES OF CHEMISTRY 



lent .parts, H 2 and CH 2 ; (3) acetylene substitution, or the exchange 

 between CH on the one hand and H 3 on the other ; and (4) carbon 

 substitution that is, the substitution of H 4 by an atom of carbon C, 

 which is founded on the -law of substitution just as is the methyl 

 substitution. These four cases of substitution render it possible to 

 understand the principal relations of the hydrocarbons. For instance, 

 the law of even numbers is 1 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 4 they are likewise even, it follows that 

 no matter how many substitutions are effected there will always be 

 obtained an even number of hydrogen atoms. 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 

 CH replaces H 3J . 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. For the largest possible quantity of hydrogen is intro- 

 duced by the methyl substitution, since it leads to the addition 

 of CH 2 ; starting from CH 4 we obtain C 2 H 6 , C 3 H 8 , and in general, 

 C n H 2)1+2 , and these contain the greatest possible amount of hydrogen. 

 Unsaturated hydrocarbons, containing 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 in 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. By means of the methylene 

 substitution alone, etkylehe, C 2 H 4 , or (CH 2 ) (CH 2 ) may be directly 

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



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 into 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 3 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, e.g. CH 3 Na. 

 These and similar products of substitution serve as a means of obtaining other more 

 complex substances from given carbon compounds. If We place the two above-named 

 products of substitution of marsh gas (metallic and haloid) in mutual contact, the metal 

 combines with the halogen, forming a very stable compound namely, common salt, 

 NaCl, and the carbon groups which were in combination with them separate in mutual 

 combination, as shown by the equation : 



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



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 the haloid 

 (hlorine) and sodium have of entering into mutual combination. 



