242 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



be tested and their volumes measured. It will be found that 

 the volume of one gas is double that of the other, and that 

 the gas of which there is the larger amount is hydrogen while 

 the other is oxygen. 



This experiment again proves what was found by synthesis, 

 viz. , that 2 vols. of hydrogen combine with 1 volume of oxygen, 

 and further, it may be noticed that these experiments also prove 

 that water is solely formed from these two gases, and contains 

 no other constituent. 



Composition of Water by Weight. We must now endea- 

 vour to find the composition of water by weight, that is, the 

 weights of oxygen and hydrogen which combine to form water. 



FIG. 126. Composition of Water by Weight. 



To do this it should be noticed that we only require the weights 

 of two out of the three substances concerned, i.e., if we know 

 the weights of hydrogen and water (or of oxygen and water), the 

 weight of the oxygen (or hydrogen) is readily calculated. The 

 experiment is done by finding the weights of the oxygen and 

 water, and for this it is best to use not oxygen itself, but some 

 oxide which readily gives up its oxygen to the hydrogen, so that 

 by weighing the oxide before and after the experiment we can 

 ascertain the weight of oxygen which it has lost. The oxide 

 used for this purpose is oxide of copper, a black powder. 



EXPT. 248. In the glass tube AC (Fig. 126) place a small 

 porcelain boat containing copper oxide, the boat and oxide being 



