56 HYDROGEN. 



air in the same manner that a bubble of common air 

 rises through water. Balloons are sometimes filled 

 with pure hydrogen gas, but more commonly carbu- 

 retted hydrogen is employed, which, as it consists in 

 great part of hydrogen, is much lighter than common 

 air. 



83. The most important of its compounds are 

 water, which is formed by its union with oxygen; 

 ammonia, a gas which it forms by combining with 

 nitrogen; and carburetted hydrogen or coal-gas, an 

 inflammable gas consisting of hydrogen and carbon. 

 The two latter will shortly come under notice (131, 

 148). 



84. As hydrogen is the lightest gas known, it is 

 often employed by chemists as a standard of com- 

 parison in expressing the relative weight of all other 

 gases. By weighing a thin glass globe filled with 

 hydrogen, and then having pumped that gas out of 

 it by means of an air-pump, and filled the globe again 

 with any other sort of gas, a second weighing gives 

 us the comparative weight or specific gravity of the 

 gas as compared with hydrogen. Suppose, for ex- 

 ample, that the globe held exactly ten grains of hy- 

 drogen and twenty-five grains of the second gas, then 

 it is plain that the latter is twice and a half as heavy 

 as hydrogen is; or, that taking equal volumes of both, 

 that of the gas would weigh twice and a half as much 

 as the hydrogen would. We should say, then, that 

 the specific gravity of that gas was 2J or 2.5, taking 

 hydrogen as the standard of unity. 



