TUE CUEMISTRY OF WATEK. HYDROGEN. 125 



erable extent, and its elements arc used in the formation of 

 almost every variety of vegetable substance. 



159. "Water of Crystallization. In the crystals of many substan- 

 ces there is considerable water. This is the case with crystals of sulphate 

 of lime, commonly called plaster of Paris. A little over one fifth by weight 

 of these crystals is water. They are perfectly dry, because the water is com- 

 bined with the substance making a part of the solid. The water is in this 

 case really solidified without freezing. Not only is it a part of the crystals, 

 but they could not be formed without it. Drive out the water by heat, and 

 the crystals fall to pieces, and you have the plaster of Paris in powder. This 

 water, thus essential to the existence of the crystals of this substance, is 

 called its water of crystallization. Burned alum is alum deprived of this 

 water by heat, and therefore its crystalline arrangement is lost. The 

 amount of water required for crystallization varies in different substances. 

 The crystals of Epsom salts, so familiar to you, are fully half water. You 

 may perhaps have noticed that sometimes some of the crystals of this salt 

 have changed into a white powder. This is 'because some of the water of 

 crystallization has escaped into the air. Any crystalline substance which 

 is apt to have this occur is said to effloresce. "When a substance has a tend- 

 ency to absorb water from the air and run to liquid, it is said to deliquesce. 

 Do not confound these terms. 



Water of crystallization is usually written separately in formulae, because 

 it seems to be less closely connected with the body than are the atoms com- 

 posing it. Thus crystallized gypsum is CaSO 4 +2H 2 O, and anhydrous 

 gypsum is simply CaSO 4 . Epsom salts is MgSO 4 +7H 2 O. 



ICO. Ammonia. Hydrogen and nitrogen united in the 

 proportion of three atoms of the former to one of the latter 

 form a colorless, alkaline, pungent gas called ammonia. Its 

 formula is therefore NH 3 . It is one of the products of the 

 decomposition of both animal and vegetable substances. 

 You therefore perceive its pungent smell in the stable ; and 

 it is also emitted from guano, the bird-manure which has for 

 many years been imported into this country from certain 

 islands. Ammonia is obtained for the purposes of commerce 

 as a secondary product in the distillation of coal. The ni- 

 trogen of the coal unites with the hydrogen to form ammo- 



