58 FIELD, FOREST AND FARM 



"No, my friend : gypsum does not contain any car- 

 bonic acid gas. It is made of lime, as in limestone, 

 but united with sulphuric acid, which heat is power- 

 less to drive out. Besides this it contains water, 

 which forms a fifth of the total weight of the stone. 

 This water, and nothing further, escapes under the 

 action of heat. With this expelled the gypsum is 

 turned to plaster. 



"But this latter has a strong tendency to take on 

 again the moisture parted with in the kiln, and thus 

 to become once more what it was in the beginning — 

 primitive stone. It is this peculiarity that renders 

 gypsum suitable for plaster. Moistened in the 

 trough, the powdery matter quickly incorporates the 

 water that is thus restored to it, and the whole hard- 

 ens into a block having the solidity of gypsum that 

 has not yet passed through the kiln. Lime turns to 

 stone by being permeated with carbonic acid gas, 

 which restores it to its limestone state. Plaster be- 

 comes stone by absorbing water, which brings it back 

 to the state of gypsum. The transformation of lime 

 is slow, of plaster very rapid. 



"As soon as it comes from the kiln plaster is 

 ground under vertical millstones and then sifted. 

 The powder must be kept in a very dry place, since 

 it contracts moisture easily and then will not harden 

 or set, as they say, when mixed with water. You 

 will perceive clearly enough that after being more 

 or less impregnated with moisture plaster cannot 

 have the same tendency to absorb the water neces- 

 sary to change it into a solid mass; the substance 



