ii: 



PHYSIOGRAPHY. 



[chap. 



to form water. Such a mixture of gases may remain for 

 any length of time, without combination being effected ; they 

 will form merely an intimate mechanical mixture of the gases, 

 but no water will be produced as long as there is no chemical 

 union. The moment, however, that a flame is applied to 

 the gases, a violent explosion occurs ; chemical combination 

 is immediately brought about, the gases cease to exist as 

 oxygen and hydrogen, contraction ensues, and water is 

 produced. If the temperature were maintained sufficiently 

 high, this water would retain the condition of steam, and it 



Flu. 2g. — Formation of water by combustion of dry hydrogen. 



would then be found that every three volumes of mixed 

 oxygen and hydrogen produced only two volumes of steam 

 or water-gas ; in other words, contraction occurs to the ex- 

 tent of one-third of the original bulk. A pint of steam consists 

 therefore of a pint of hydrogen and half a pint of oxygen in 

 a state of chemical combination, whereby the pint and a half 

 of mixed gases becomes condensed to a pint of water-gas. 

 But, at ordinary temperatures, the steam rapidly condenses 

 to a liquid ; a cubic foot of steam condensing to about a 

 cubic inch of water. If oxygen and hydrogen be exploded 



