Fire and Heat 121 



air. It is not only lighter than air, but it has less density than 

 any other known substance. Because of this fact it is taken 

 as the standard of density for all gases. These are what we 

 consider its physical properties, that is, properties which appeal 

 to our senses. 



The wonderful lightness of hydrogen is the reason for its 

 use especially in experimental balloons which ascend to high 

 altitudes. It is, however, expensive to produce and tends to 

 leak through silk. Consequently illuminating gas, which is 

 cheaper and does not escape rapidly through silk, is generally 

 used in ordinary airships and balloons. 



Hydrogen has other properties, as your experiments show. 

 It unites very freely with oxygen. When mixed with oxygen 

 and exposed to flame its union is sudden and explosive. Al- 

 though it does not support combustion, it is itself combustible, 

 burning in oxygen with a pale, bluish, intensely hot flame. 

 This indicates that hydrogen is very active. The heat which 

 it produces is so intense that in the oxy-hydrogen flame it 

 readily cuts, as if with an extremely sharp knife, a steel bar 

 two feet thick. A given weight of hydrogen burning in oxygen 

 produces a greater amount of heat than an equal weight of any 

 other two known substances. Its heat value (page 143) is more 

 than four times that of pure carbon. The product of burning 

 hydrogen is water. And the heat evolved by a given weight 

 of hydrogen combining with a given weight of oxygen is suffi- 

 cient to raise through one Centigrade degree of temperature 

 about four thousand times its own weight of water. To break 

 water down into its constituents requires exactly as much heat 

 as is produced by them in uniting. Because of its inflammable 

 nature one might not expect to find hydrogen free in the air ; 

 but it does exist in traces ; in your experiments you probably 

 allowed some to escape. 



Hydrogen in compounds. The importance of hydrogen 

 in life may be more fully appreciated when one recognizes 

 the large number of compounds in which it is an essential 



