THE CHEMISTRY OF WATER. HYDROGEN. 



117 



Hydrogen. 



Platinum. 



equal quantities, by 

 weight, of these two 

 substances, as well as 

 of water and air. Air 

 is about fourteen and 

 a half times as heavy 

 as hydrogen, water 

 more than eleven 

 thousand times, and 

 platinum nearly a 

 quarter of a million 

 times. Here is a tab- 

 ular statement of the relative gravities of these substances: 



Fig. 39. 



In the first column, taking hydrogen as 1, the proportionate 

 weights of the other substances are given. In the second 

 column we call air 1, and in the third water. 



146. Hydrogen and Carbonic Anhydride Contrasted. Car- 

 bonic anhydride'is twenty-two times as heavy as hydrogen. 

 It is so much heavier than air that you can set a jar of it 

 down with its mouth open, and the gas will remain in it for 

 some time. Its weight tends to keep it in the jar, and it 

 will only gradually escape by its disposition to mingle with 

 other gases, as noticed in 120. But if you set down a jar 

 of hydrogen in this way, it rises out of the jar at once, 

 precisely as oil would rise out of a jar plunged into wa- 

 ter with its mouth upward. In order to keep the hydro- 

 gen in the jar it must be held with its mouth downward. 

 We can follow the contrast farther. Carbonic anhydride 



