HEAT PHENOMENA. 235 



can be calculated; for the combination 



2H + = H^O (solid) liberates 70,400 units of heat, 

 Si + 20 =: SiO^ Hberates 21 1,100 units of heat, 



and therefore 



2Si + 4H20 = 2SiO' + 8H liberates 140,600 units. 



The molecular weight of the andesine feldspar under discussion is 757.6, 

 and the heat liberated per unit of weight would be 



140,600 1 g. ., 

 ^^ = 186 units. 



The specific heat of feldspars varies from 0.183 to 0.196. If the ande- 

 sine under discussion is supposed to have a specific heat of 0.186, the tem- 

 perature resulting from the substitution of hydrogen for silicon would be 

 1000° C. 



The aH'o is water of hydration. — If, thcu, thc watcr iu kaolin wcrc chemically 

 combined, a temperature would be produced much above that known to be 

 sufficient to expel the water from clay, and the only inference I can draw is 

 that the water is not, as has sometimes been maintained, chemically combined, 

 but is merely water of hydration. The latter view (which is also generally 

 held) is further supported by the varying amounts of water which various 

 analysts have found in kaolin. As is well known, Tschermak even denies 

 that kaolin is a product of the decomposition of plagioclase, affirming that 

 the resulting hydrated aluminium silicate contains but a single molecule of 

 water. 



Nothing known of the heat of hydration of kaolin. The pUrpOSC of the foregoiug argu- 

 ment is to show that if any considerable quantity of heat is evolved during 

 kaolinization, it must in all probability be due to the simple hydration of 

 aluminium silicate. But of the heat liberated by the hydration of salts little 

 is known, except (1) that the quantity is usually small, (2) that it is some- 

 times negative, and (3) that the diff'ereut molecules of water combine with 

 difi'ering amounts of energy, indicating that the nature of their union difi"ers. 

 Of the heat of hydration of kaolin we know nothing specific, nor am I 



