186 CHEMICAL AFFINITY. 



Order of affinity. The affinity between bodies appears to be 

 of different degrees of intensity. Lead, for instance, has cer- 

 tainly a greater affinity than silver for oxygen, the oxide of the 

 latter being easily decomposed when heated to redness, while 

 the oxide of the former may be exposed to the most intense 

 heat without losing a particle of oxygen. Again, it may be in- 

 ferred that potassium has a still greater affinity for oxygen than 

 lead possesses, as we find the oxide of lead easily reduced to the 

 metallic state when heated in contact with charcoal, while po- 

 tash is decomposed in the same manner with great difficulty. 

 But the order of affinity is often more strikingly exhibited in 

 the decomposition of a compound by another body. Thus 

 sulphuretted hydrogen gas is decomposed by iodine, which 

 combines with the hydrogen forming hydriodic acid, and libe- 

 rates sulphur. The affinity of iodine for hydrogen is, therefore, 

 greater than that of sulphur for the same body. But hydriodic 

 acid is deprived of its hydrogen by bromine, and hydrobromic 

 acid is formed; and this last is decomposed in its turn by 

 chlorine, and hydrochloric acid produced. It thus appears 

 that the order of the affinity of the elements mentioned for 

 hydrogen is, chlorine, iodine, bromine, sulphur. The order of 

 decompositions, in the precipitation of rnetals by each other 

 from their saline solutions, also indicates the degree of affi- 

 nity. Thus from the decomposition of the nitrates of the 

 following metals, the order of their affinity for nitric acid 

 and oxygen may be inferred to be as follows : zinc, lead, 

 copper, mercury, silver; zinc throwing down lead from the 

 nitrate of lead, and all the other metals which follow it, lead 

 throwing down copper ; copper, mercury ; and mercury, silver ; 

 while nitrate of zinc itself is not affected by any other metal, and 

 nitrate of silver is decomposed by all the metals enumerated. 

 Bodies were first thus arranged according to the degree of their 

 affinity for a particular substance, inferred from the order of 

 their decompositions, by Geoffroy and Bergman, and tables of 

 affinity constructed of which the following is an example. Order 

 of affinity of the alkalies arid earths for sulphuric acid. 



Barytes 



Strontian 



Potash 



Soda 



Li ni c 



Magnesia 



Ammonia 



