226 MINERALOGY 



9 H 2 O and form a white powder, Na^COs, H 2 O. In certain cases 

 the water of crystallization when driven off may be reabsorbed from 

 a damp air. Gypsum, CaS04, 2 H 2 0, when heated to 130 C. loses 

 one of the molecules of water and when powdered forms plaster of 

 Paris ; this will gradually pass back to CaS0 4 , 2 H 2 O by the absorp- 

 tion of water from a damp atmosphere, or it will take it up very 

 quickly when mixed with water, setting into a hard mass, and at 

 the same time evolving heat as the water is combined. This heat 

 of combination will vary with the molecule of water given off and 

 is the greatest in case of the last molecule to be liberated from a 

 compound, or the one which is liberated at the highest temperature. 

 The heat of combination is a measure of the bond. 



Water which is driven off at high temperatures is termed water 

 of constitution, and its relation to the crystalline molecule is usually 

 considered to be entirely different from that of the water of crystal- 

 lization. This difference is not only indicated by the heat of com- 

 bination, but when the compound is dissolved, the water of consti- 

 tution is found to occupy a much smaller volume than the water 

 of ' crystallization, which is of exactly the same volume as the 

 water of the solvent, or the water in which the salt is dissolved. 

 Also the change of volume in case of fusion is much smaller with 

 the water of constitution. 



Water of constitution, when written in the mineral formula, 

 to indicate this difference, is incorporated with the chemical 

 elements, and is not written as water after the formula, as is the 

 case with the water of crystallization. Epsomite, MgSO4, 7 H 2 0, 

 will lose 6 H 2 O at 100 C. ; the seventh molecule is not separated 

 until 210 C. is reached. The last molecule must be combined 

 in some other or different way, and when combining with mag- 

 nesium sulphate this seventh molecule will liberate twice as much 

 heat. To indicate this difference in bond of this single molecule 

 of water, the formula of magnesium sulphate may be written 

 (Mg.OH)HSO 4 , 6H 2 0. The water of constitution may be con- 

 nected with the metal or base, forming basic water or basic salts, 

 or it may be connected with the acid, forming acid salts. 



In all acids the acid radicle is combined with hydrogen as 



TT O \ 



TT f\ /SO 2 , sulphuric acid. In the formation of salts this 

 11 U / 



acid hydrogen is replaced with a metal. When all the hydrogen is 

 replaced, -^ a Q/SO 2 , the salt is said to be normal, or, as rep- 



