WATER. 261 



through which the gas has then to pass is so great as com- 

 pletely to intercept the passage of flame. 



Hydrogen is capable of forming two compounds with oxygen, 

 namely water, which is the protoxide, and the peroxide of 

 hydrogen. 



Uses. The most important of the present applications of 

 hydrogen gas is in the oxi-hydrogen blow-pipe. It has been 

 superseded as a material for inflating balloons, by coal gas, the 

 balloon being proportionally enlarged to compensate for the less 

 buoyancy of the latter gas. 



PROTOXIDE OF HYDROGEN. WATER, 



Equivalent 1 1 2.5, or 9 on hydrogen scale ; formula H + O, or 

 H O ; density 1, as steam 620.2 (air 1000) ; combining measure 

 of steam \ I I. 



Mr. Cavendish first demonstrated, in 1781, that the product 

 of the combustion of hydrogen and oxygen is water. He 

 burned known quantities of these gases in a dry glass vessel, 

 and found that water was formed in quantity exactly equal to 

 the weights of the gases which disappeared. It was afterwards 

 established by Humboldt and Gay-Lussac, that the gases unite 

 rigorously in the proportion of two volumes of hydrogen to one 

 volume of oxygen, and that the water produced by their union 

 occupies, while it remains in the state of vapour, exactly two 

 volumes (page 131). The proportion of the constituents of 

 water by weight was determined with extraordinary care by 

 Berzelius and Dulong. Their method was to transmit dry 

 hydrogen gas over a known weight of the black oxide ot 

 copper, contained in a glass tube, and heated to redness by a 

 lamp. The gas was afterwards conveyed through another 

 weighed tube containing the hygrometric salt, chloride of 

 calcium. The hydrogen gas in passing over the oxide of copper, 

 combines with its oxygen and forms water, which is carried 

 forward by the excess of hydrogen gas, and absorbed in the 

 chloride of calcium tube. The weight of this water being ascer- 

 tained, the proportion of oxygen it contains is determined by 

 ascertaining the loss which the oxide of copper has sustained ; 

 the difference is the hydrogen. The mean of three such ex- 

 periments gave as the composition of water : 



