EFFECTS OF GYPSUM. 115 



previously explained under lime, some physical or 

 chemical defect which prevents their action. 



Gypsum, before it is burned, consists of sulphuric 

 acid, lime, and water; of the latter, there are about 

 21 lbs. in every hundred. This water can be easily 

 driven off by heating the ground gypsum. This may 

 be done with a small quantity, by way of experiment, 

 over a common lamp. During heating, it whitens : 

 it is this burned gypsum that is used for the cornices 

 of rooms, for making casts, for hard finish, etc. When 

 water is mixed with it, a considerable degree of heat 

 is produced, the 21 per cent of water is again ab- 

 sorbed, becoming once more a part of the solid stone, 

 and the whole mass hardening or setting, as it is 

 termed, in a few moments. It is upon this property 

 of hardening when mingled with water, that the uses 

 of gypsum in the arts, as above mentioned, depend. 



This manure frequently produces a most beneficial 

 effect when applied as a top dressing upon pastures 

 and meadows : it is also a favorite and excellent appli- 

 cation to young corn and potatoes. It is of service 

 not only by the valuable nutriment which it furnishes 

 to the plant, but also from a certain power which it 

 possesses of absorbing moisture and gases. 



a. Liebig has supposed that much of its effect upon 

 grass land is owing to this property, that it attracts 

 ammonia from the atmosphere, and retains it for the 

 use of plants. This is without doubt an important 

 effect, but should not be considered the principal one. 



b. To this same property is to be ascribed its action 

 when scattered over compost heaps, or mixed into 

 the liquid in tanks. In both cases it absorbs ammo- 

 nia, and prevents its escape. White fumes of ammo- 

 nia may sometimes be perceived, both by the eye and 

 the sense of smell, rising from the surface of ferment- 

 ing manure heaps. A little gypsum sprinkled over 

 the surface of the heap, will arrest this evaporation 

 and loss almost immediately. 



