APPENDIX V. 



WATER OP CRYSTALUZATION. 

 Brucite (Magnesium Hydrate). Mg(0H)2 or MgO'HzO. (Fig. 130.) 



This mineral was studied with reference to the question of the effect 

 of the OH-group. It is also of interest because it does not contain 

 carbon. The common massive foliated variety was used, from which 

 a thin, clear lamina was removed. 



The transmission curve is conspicuous for a very large minimum at 

 2.5 [I, beyond which there are no bands until we arrive at 9 /t, beyond 

 which point there is complete opacity. The characteristic band of 

 carbohydrates, at 3.1 to 3.43 [x, is absent; in fact, there are no bands in 

 the whole spectrum which are to be found in the hydrocarbons. 



This is the first compound studied which has a large absorption band 

 situated near the visible spectrum.^ In fact, it is but the second com- 

 pound discovered which has a large band in this region, the first min- 

 eral being beryl, HgBeeAl^SioOg^, which Konigsberger^ found to have 

 a large band at about 0.86 fx. 



The question of the condition of the water in these different com- 

 pounds is of interest. By " water of constitution " is meant that the 

 HoO is chemically combined with the other constituents of the molecule. 

 On heating, water will be given off, but the residue will not take up 

 water when placed in it. This is not unlike the combustion of a carbo- 

 hydrate, which contains O atoms. Brucite, beryl, tourmaline, and mica 

 are examples of this class of compounds. On the other hand, in crystals 

 having " water of crystallization " the molecules of water and of the 

 mineral are thought to exist in their entirety. Here water is also 

 given off on applying heat, e. g., ordinary copper sulphate, but in these 

 compounds water zvill again be taken up. Gypsum (CaS04 -f 2H2O), 

 which has been discussed with water, belongs to the latter class of 

 compounds. 



The curves of gypsum, which has " water of crystallization," and 

 of brucite and of mica, which have " water of constitution," are radi- 

 cally different. The former has the bands found in water, the latter 



^Konigsberger: Ann. der Physik (4), 4, p. 796, 1901, calls attention to the fact that 

 no substances are known which have large absorption bands near the visible spectrum. 

 ^Konigsberger : Ann. der Physik, 61, 687, 1897. 



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