306 SALTS. 



water, and then pouring it into moulds. A coin may 

 be copied by pouring such a paste into a small paper box 

 containing the coin. Two parts of ordinary ground 

 gypsum, heated moderately until vapor ceases to escape, 

 and then mixed with one part of water, form a good 

 proportion. The heat should not be carried very far 

 beyond that of boiling water, or the plaster refuses to 

 set. 



772. The hardening of the plaster part 

 ter casts hard- takes place very rapidly. It is owing to 



the re-combination of the material with 

 water. The water thus absorbed exists in a solid form 

 in the compound, as in other salts. 



773. ALUMINATED PLASTER. Harder and 



What is alu- 



minatedplas- better casts, more nearly resembling mar- 

 ble, are made by steeping the burned gyp- 

 sum for six hours in strong alum water, and then re- 

 heating it at a higher temperature. After being again 

 pulverized, it may be used like ordinary plaster, but 

 requires more time to harden. 



774. SULPHATE OF SODA. GLAUBER'S 



Describe sul- .-.,, . , . > r 



pkateof soda, SALT. This is a white salt, forming crys- 

 and itsprcpa- ta } s belonging to the third system, such as 



ration. J 



are represented in the fig- 

 ure. It is used to some extent in medi- 

 cine, and in large quantities for the pro- 

 duction of carbonate of soda. It is prepared by pour- 

 ing oil of vitriol upon common salt. A double decom- 

 position takes place between the salt and the water of 

 the acid ; hydrochloric acid is formed, which passes off, 

 and soda, which remains combined with the sulphuric 



