188 CHKMICAL AFFINITY, 



red heat, provided a current of air or of steam is passing over 

 it which may carry off the carbonic acid gas, but the decom- 

 position ceases when the carbonate is surrounded by an 

 atmosphere of its own gas ; and the carbonate may even be 

 heated to fusion, in the lower part of a crucible, without de- 

 composition. Here the occurrence of decomposition depends 

 entirely upon the existence of a foreign atmosphere into which 

 carbonic acid can diffuse. Nitrates of alumina and peroxide of 

 iron in solution, are decomposed by the spontaneous evapora- 

 tion of their acid, even at the temperature of the air ; and so is 

 an alkaline bicarbonate when in solution, but not when dry. 

 A change in the composition of the gaseous atmosphere may 

 affect the order of decomposition as in the following cases : 



When steam is passed over iron at a red heat a portion 

 of it is decomposed, oxide of iron being formed and hy- 

 drogen gas evolved. From this experiment it might be in- 

 ferred that the affinity of iron for oxygen is greater than that 

 of hydrogen. But let a stream of hydrogen gas be conducted 

 over oxide of iron at the very same temperature, and water 

 is formed, while the oxide of iron is reduced to the me- 

 tallic state. Here the hydrogen appears to have the greater 

 affinity for oxygen. But the result is obviously connected 

 with the relative proportion between the hydrogen and steam 

 which are at once in contact with the metal and its oxide at a 

 red heat. When steam is in excess, water is decomposed, but 

 when hydrogen is in excess, oxide of iron is decomposed ; and 

 why, because the excess of steam in the first case is an atmos- 

 phere into which hydrogen can diffuse, and the disengagement 

 of that gas is therefore favoured ; but in the second case the 

 atmosphere is principally hydrogen, and represses the evolution 

 of more hydrogen, but facilitates that of steam. The affinity 

 of iron and hydrogen for oxygen at the temperature of the ex- 

 periment, is so nearly balanced that the one affinity prevails 

 over the other, according as there is a proper atmosphere into ' 

 which the gaseous product of its action may diffuse. This 

 affords an inteUigible instance of the influence of mass or quan- 

 tity of material, in promoting a chemical change ; the steam or 

 the hydrogen, as it preponderates, exerting a specific influence, 

 in the capacity of a gaseous atmosphere. 



The remarkable decomposition of alcohol by sulphuric acid, 

 which affords ether, is another similar illustration of decom- 



