The Chemical Relations of Water 303 



reference to air and water as examples of each class 

 respectively. 



Air is a mixture because it may be made by simply 

 passing volumes of nitrogen and oxygen, which bear the 

 same relation to one another as do the volumes of these 

 gases found in air, into an empty vessel. The mixture 

 behaves in every way like air. On the other hand, 

 if two volumes of hydrogen and one volume of oxygen 

 be mixed, no water is produced by the mixing, and the 

 mixture behaves neither like water nor steam, since 

 the insertion of a flame causes an explosion. 



In the formation of air by the mixing of nitrogen 

 and oxygen, no heat is evolved. The evolution of heat 

 is a common occurrence during chemical combination, 

 and the great quantity of heat produced when the 

 constituent elements of water are caused to unite to 

 form water offers a striking contrast to the absence 

 of heat production in the case of air. 



Again, no change of volume follows the admixture 

 of nitrogen and oxygen, while, as we have seen, the 

 steam formed by the combination of hydrogen and 

 oxygen only occupies two-thirds of the combined 

 volumes of the mixed gases. Change of volume is 

 another frequent concomitant of chemical action. 



The physical properties of air are the mean of those 

 of the gases nitrogen and oxygen. Thus its specific 

 gravity may be calculated from the ratio by volume 

 in which these two gases occur, when the specific 

 gravities of the gases are known. The specific gravity 

 of steam is not the mean of those of hydrogen arid 

 oxygen, as calculated, but is considerably greater. 

 Similarly, the other physical properties of water cannot 

 be foretold from a consideration of those of its con- 

 stituents. 



