j 1 4- CHEMICAL PROPERTIES AND COMPOSITION 



in question. Positive influence acts in the opposite direc- 

 tion. Hence these opposing influences have first to be 

 dealt with in the case of each element. 



i. Influence of Positive Elements, 

 (a) Influence of Hydrogen. 



(a) Hydrogen affects acidity. Beginning with the simplest 

 case, it is first to be noted that the influence of hydrogen 

 in question is appreciable in carbon itself. The element 

 named has a certain tendency to combination with the 

 metals : the compounds with iron (in cast iron) and nickel 

 are quite stable bodies which are decomposed by acids, but 

 not by water : and the compounds with the lighter metals, as 

 calcium (carbide), can be produced directly, but are decom- 

 posed by water with formation of acetylene. It is different 

 when the carbon valencies are as far as possible saturated 

 with hydrogen, as in methyl : the tendency to combination 

 with the metals is then so far reduced that, e.g. zinc 

 methyl is instantaneously decomposed by water, and indeed 

 spontaneously inflammable, whilst compounds with the 

 light metals are so difficult to arrive at that they have not 

 yet been prepared. 



In more complicated cases the same thing is observed ; 

 compare compounds that contain the groups : 



HCEEC, HC = CH, H 2 C = CH, and H 3 C CH 2 . 



In the first the tendency to take up metals is so far 

 marked that the acetylene compounds (acetylides) show a 

 weak acid character, and give precipitates containing metal, 

 with silver oxide and cuprous oxide. So do acetylene and 

 propargyl alcohol, forming 



Ag C=C H and Ag C = C CH 2 OH. 



This tendency disappears in the three other groups men- 

 tioned, which are richer in hydrogen ; only in the group 

 HC = CH is an influence in the same direction observed '. 



1 Henrich, Centralblatt, 1899, n. 701. 



