ii8 CHEMICAL PROPERTIES AND COMPOSITION 



(/3) Examples from Inorganic Chemistry. The same 

 effect of the introduction of metals is seen in the pro- 

 gressive saturation of polybasic acids such as phosphoric, 

 in which the successive evolutions of heat diminish notably. 

 P0 4 H, + NaOH 14-8, +NaOH 12-3, +NaOH 7. 



The last number is, however, not quite accurate, as on 

 account of the hydrolysis of trisodium phosphate, PO 4 Na 3 , 

 for complete saturation the number should be 8-3 instead 

 of 7. But both the small value and the fact of hydrolysis 

 show that the introduction of sodium affects the tendency 

 to take up metal. 



The same thing is observed in the case of sulphuretted 

 hydrogen, SH 2 , which, though containing two hydrogen 

 atoms, only forms NaSH in aqueous solution even with 

 excess of soda, the compound Na 2 S being converted into 

 the same on solution in water. 



Na 2 S + H 2 O = NaSH + NaOH. 



Still more striking is the behaviour of sodium oxide, 

 Na.,0, which is even decomposed by hydrogen : 



Na 2 O + H = NaOH + Na. 



(y) Effed of Distance. It may be added that distance 

 plays a part in the influence of sodium also ; in this 

 connexion we may recall the behaviour of fumaric and 

 maleic acids as described in Part II, p. 123, in that the 

 introduction of the first sodium atom into maleic acid, 

 although the stronger acid of the two, exercises a greater 

 influence on the second carboxyl group, because nearer than 

 in fumaric acid ; consequently maleic acid is less suited to 

 take up a second atom of sodium than fumaric. 



(c) Influence of the Heavy Metals, especially Silver. 



Whilst introduction of a light metal hinders its further 

 introduction, that is not the case with the heavy metals 

 such as silver, in accordance with p. 90, but rather the 

 .reverse. In this may lie partly the explanation of the fact 



