BARIUM, STRONTIUM, AND CALCIUM. 231 



dry substance. The hardening takes place quite rapidly. 

 The moistened plaster can also be moulded into casts, plas- 

 ter heads, ornamental work for walls, called stucco-work, 

 etc. It is remarkable that, in making the gypsum anhy- 

 drous, if the heat be carried above a certain point, its affin- 

 ity for water will be destroyed, and there will be no " set- 

 ting" of the plaster. 



320. Casts of Coins. Copies of coins and medals can be 

 taken very readily with the moistened plaster. For this 

 purpose put the coin into a paper box, or, if you have not 

 one of the proper size, fasten a slip of paper around the 

 coin, securing the loose end by a little sealing-wax, and 

 pour the plaster in upon the coin. After a few minutes it 

 will become so*hard that both the paper and the coin can 

 be removed. A reversed impression will be formed on the 

 under surface of the plaster. To get from this a real copy 

 of the coin, smear the impression with a very little of a 

 strong solution of soap, having a few drops of oil mixed 

 with it, and then pour upon it some of the plaster. 



The use of gypsum in agriculture will be spoken of in 

 another part of this book. 



321 . A Singular Case. If sulphate of calcium (gypsum) and carbon- 

 ate of ammonium be mingled together in solution, there will result car- 

 bonate of calcium, or chalk, and sulphate of ammonium. Now if we take 

 these two substances thus resulting, and, powdering them finely, mix them 

 together, and expose the mixture to a red heat in a close vessel, we shall 

 have the original sulphate of calcium and carbonate of ammonium pro- 

 duced again. Here we have heat occasioning a chemical process exactly 

 the reverse of that caused in a solution at an ordinary temperature. 



322. Chloride of Lime. The salt which sometimes goes 

 by this name, and sometimes by the name of bleaching 

 powder, is a white powder, having the odor of chlorine gas, 

 because this gas escapes from it continually in a small 

 amount. The reason of its escape is that the carbonic an- 



