HYDROGEN. 255 



for the cost of the oxygen. Where a large quantity of oxygen 

 is required, as in this application of it, the gas may be obtained 

 by heating oxide of manganese in a cylinder of cast iron sup- 

 ported over a furnace, like the retort for coal gas. The calcined 

 oxide does not regain its oxygen when afterwards exposed to the 

 air, as was once supposed,but would still be of some value in the 

 preparation of chlorine. 



SECTION II. 

 HYDROGEN. 



Equivalent 12.5 (oxygen 100), or 1 as the bases of the hydro- 

 gen scale ; symbol H ; density 6*9 (air 1000) ; combining 

 measure \ \ \ (two volumes.) 



Hydrogen gas, which was long confounded with other in- 

 flammable airs, was first correctly described by Cavendish, in 

 1766. It does not exist uncombined in nature, at least the 

 atmosphere does not contain any appreciable proportion of 

 hydrogen. But it is one of the elements of water, and enters 

 into nearly every organic substance. Its name is derived from 

 v&u,o, water, and yewaw, I generate, and refers to its forming 

 water when oxidated. 



Preparation. This element, although resembling oxygen in 

 being a gas, appears to be more analogous to a metal in its 

 chemical properties. By heating oxide of mercury, we have 

 seen it resolved into oxygen and mercury ; and several other 

 metallic oxides, such as those of silver and gold, are susceptible 

 of a similar decomposition. But some others are deprived of 

 only a portion of their oxygen by the most intense heat, such 

 as peroxide of manganese ; and many, such as the protoxide of 

 lead, are not decomposed at all by simple calcination. By 

 igniting the latter oxide, however, mixed with charcoal, its 

 oxygen goes off in combination with carbon, as carbonic oxide, 

 and the lead is left. The oxide of hydrogen or water is in the 

 same case. Heat alone does not decompose it. But potassium 

 and sodium brought into contact with it, at the temperature of 

 the air, combine with its oxygen, and are converted into the 

 oxides potash and soda ; and hydrogen is consequently liberated, 

 water being the oxide of hydrogen. 



