WATER 



67 



stable substance. When steam is superheated, hardly a trace of 

 decomposition occurs. Even when the temperature reaches 

 2000 (far above a white heat), only 1.8 per cent of the vapor is 



FIG. 35 



broken up into oxygen and hydrogen. When the steam cools, 

 these elements recombine: 



Water < Hydrogen + Oxygen. 



Hydrates. Many substances unite with water to give 

 compounds called hydrates. Thus if we take zinc sulphate (p. 52) 

 and dissolve it in water and allow the excess of the latter to evapo- 

 rate, the solid appears in long transparent crystals. When these 

 are dried with blotting paper and heated in a test-tube, they give 

 off a large amount of steam. The hydrate of zinc sulphate decom- 

 poses and leaves anhydrous (Greek, deprived of water) zinc sul- 



