104 PHYSIOGRAPHY. [chap. 



If a cubic inch of oxygen is generated, two cubic inches of 

 hydrogen will be obtained in the same time ; and it is 

 found that these proportions are exactly preserved, where- 

 ever and whenever water is subjected to decomposition. 

 It is seen, then, not only that water is composed of the 

 two substances, oxygen and hydrogen, but that they exist 

 in water in a constant proportion ; so that, when they are 

 set free and assume the gaseous state, there is always one 

 volume of oxygen to two volumes of hydrogen gas. 



This experiment gives a clear insight into the essential 

 constitution of water. None of the changes which were 

 described in preceding chapters had in any way affected 

 this constitution. Water may be frozen, for example, into 

 solid ice, but the ice will consist of oxygen and hydrogen 

 in exactly the same proportions as in the liquid water. 

 The water may be boiled and become the invisible gas, steam, 

 but the steam will consist of oxygen and hydrogen in exactly 

 the same proportions as in the water or in the ice. It will 

 thus be understood that the physical properties of matter 

 may be altered without affecting its deeper chemical con- 

 stitution. The three conditions of a solid, a liquid, and 

 a gas — represented respectively by ice, water, and steam 

 — are physical states dependent mainly on temperature, and 

 the chemical constitution of steam remains unaltered by a 

 temperature far above the boiling point, while that of ice 

 is not affected by any known degree of cold. It frequently 

 liappens, however, that the application of heat, instead 

 of effecting merely physical change, produces chemical 

 alteration in the substance. This was the case, it will 

 be remembered, with the red oxide of mercury referred to 

 in the last chapter (p. 77) ; when heated it did not melt, or 

 fusC; or liquefy, but was at once split up into its constituents 

 — mercury and oxygen. And, in like manner, under certain 



