D. CHEMISTRY AND METALLURGY. 225 



ops the crystals takes up in solution considerable calcium 

 sulphate. Third, a portion of this water being evaporated 

 by the heat resulting from the chemical combination, a crys- 

 tal is formed which determines the crystallization of the 

 whole mass, just as when a crystal of sodium sulphate is 

 dropped into a supersaturated solution of that salt. It is 

 not, however, until after some time that the mass acquires 

 its maximum hardness, the plaster then containing the re- 

 quired proportion of water, i. e., two molecules to one of the 

 calcium sulphate. This amount of water does not lessen by 

 evaporation. In mixing plaster, only about 12 per cent, of 

 water should be added, as ordinary plaster itself contains 

 about 8 per cent. ; but in actual practice the amount used 

 is never less than 33 per cent. This excess is added in or- 

 der to prevent setting of the mass before it can be used. 

 But the effect is injurious, since very porous, slowly drying 

 plasters are produced in this way, which rapidly determine 

 nitrification. To diminish the rapidity of setting is to delay 

 the crystallization, which can be effected by adding gum, 

 gelatin, guimauve powder, glycerine, and similar bodies. In- 

 ert substances, like sand and barium sulphate, for example, 

 on the other hand, simply diminish the solubility of the ma- 

 terial, without in the least retarding the setting process. 

 Overburned plasters may be utilized by admixture with 

 ordinary plaster, since the crystallization of the latter ex- 

 tends to the former, and occasions the setting of the entire 

 mass. A similar effect is produced by simply mixing the 

 two plasters together. Lime acts favorably upon plaster, as 

 it not only increases the rapidity with which it sets, but it 

 gives it an additional hardness. Plasters to which 10 per 

 cent, of lime has been added are capable of taking a polish. 

 Samples have been made containing as high as 75 per cent, 

 of lime ; they are hard and light, and may yet serve some 

 useful purpose in the arts. 6 B, LXXLX., 658. 



CONSTITUTION OF GUM TRAGACANTH. 



Giraud has made a minute examination of the chemical 

 characters of gum tragacanth. He finds (l) that this gum 

 is but very slightly soluble in water, and that the product in 

 the filtrate is not a definite principle like arabin, but is a 

 mixture of several substances; (2) that digested on the wa- 



K2 



