COMMON ELEMENTS OF THE EARTH 149 



stereopticons. The lightness of hydrogen makes it useful in 

 filling balloons. 



160. The Composition of Water. Whenever hydrogen 

 is burned in oxygen or in air, it unites with the oxygen and 

 forms a vapor, which on cooling condenses to a liquid. This 

 liquid is pure water. On the other hand, by an electric 

 current (Fig. 77) water may be decomposed into two gases, 

 which, on being tested, prove to be oxygen and hydrogen. 

 When the volumes of these two gases are measured, it is found 

 that there is twice as much hydrogen as oxygen. Weighing 

 the gases shows that the oxygen weighs eight times as much 

 as the hydrogen. Thus it is shown that water is composed of 

 two volumes of hydrogen to one of oxygen, or one unit of 

 weight of hydrogen to eight of oxygen. From these measure- 

 ments it is seen that when equal volumes of these gases are 

 considered, oxygen weighs sixteen times as much as hydrogen. 



161. Nitrogen. The gas nitrogen can best be described 

 by negatives. It does not burn as hydrogen does, nor aid 

 in burning as does oxygen, and it does not readily unite with 

 other substances to form compounds. But there are many 

 compounds of nitrogen which are far from negative in 

 character. Combined with hydrogen, it forms the pungent 

 gas ammonia. With hydrogen and oxygen, it makes nitric 

 acid, which dissolved metals. The explosives dynamite, 

 nitroglycerine, and gunpowder contain compounds of 

 nitrogen. 



Compounds of nitrogen are necessary in the food of ani- 

 mals; these compounds are obtained from plant food or 

 from the bodies of animals that have eaten plant food. 

 Plants obtain a large amount of their nitrogen from com- 

 pounds of nitrogen in the soil. 



Air is a mixture of nitrogen and oxygen in the proportion 

 of about four volumes of nitrogen to one of oxygen, to- 

 gether with a very small proportion of carbon dioxide, 

 water vapor, and some other gases. 



