256 HYDROGEN. 



Iron and many other metals decompose water, and become 

 oxides, at a red heat. Hence, hydrogen gas is sometimes 

 procured by transmitting steam through an iron tube placed 

 across a furnace and heated red-hot. Some other compounds 

 of hydrogen are decomposed more easily than water, by iron and 

 zinc. The chloride of hydrogen or hydrochloric acid is decom- 

 posed by these metals, and evolves hydrogen, at the ordinary tem- 

 perature of the air. But this gas is more generally obtained by 

 putting pieces of zinc or iron into oil of vitriol or the concen- 

 trated sulphuric acid, diluted with 6 or 8 times its bulk of 

 water. The hydrogen is then derived from the decomposition 

 of the proportion of water intimately united with the acid, as 

 illustrated in the following diagram, zinc being used, and the 

 quantities expressed : 



Before decomposition. After decomposition. 



6 J3| oil of vitriol, f Hydrogen. . 12| . . . . 12| hydrogen. 

 < 



or sulphate < Oxygen 

 of water. . I Sulphuric acid 501 

 403 zinc . . . Zinc . . .403 - 1004 sulphate of 



- oxide of zinc. 



ioi6j 



Or by symbols : 



H O-f S O 3 and Z = Z O-f S O 3 and H. 



The zinc dissolves in the acid with effervescence, from the 

 escape of hydrogen gas. It will be observed that the products 

 after decomposition, mentioned in the last column, hydrogen 

 and sulphate of oxide of zinc are similar to those before de- 

 composition, in the first column, zinc and sulphate of water ; 

 and that the change occurring is simply the substitution of zinc 

 for hydrogen in the sulphate of water. The large quantity of 

 water used with the acid is useful to dissolve the sulphate of 

 zinc formed. 



Zinc is generally preferred to iron, in the preparation of hy- 

 drogen, and is previously granulated, by being fused in a 

 stone-ware crucible, and poured into water ; if sheet zinc be 

 used, it must be cut into small pieces. The common glass 

 retort may be used in the experiment, or a gas-bottle, such as 

 the half pound phial (Fig. 33), with a cork having two perfora- 

 tions fitted with glass tubes, one of which descends to the 



