SPECIFIC FORM. -.289 



cepti'onal phenomenon. It is, on the contrary, quite 

 frequently observed that the different faces of a 

 crystal show marked differences in solubility. This 

 is what happens in every case for the mutilated face 

 in comparison with the others ; the matter is less 

 soluble there. The consequence of this is clear ; the 

 growth must preponderate on that face, since there 

 the mother liquor will become super-saturated beforS 

 being super-saturated for the others. We may 

 explain this result in another way. Each face of the 

 crystal in contact with the mother liquor is exposed 

 to two antagonistic actions : The matter deposited 

 upon a surface may be taken away and redissolved if, 

 for any reason whatever, such matter becomes more 

 soluble than that of the liquid stratum in contact 

 with it ; in the second place, the matter of this liquid 

 stratum may, under contrary conditions, be deposited? 

 and thus increase the body of the crystal. There is, 

 then, for each point of the crystalline facet, a positive 

 operation of deposit which results in a gain, and a 

 negative operation of redissolution which results in a 

 loss. One or the other effect predominates according 

 as the relative solubility is greater or less for the 

 matter of the facet under consideration. On the 

 mutilated surface it is diminished, deposition then 

 prevails. 



But this is only the immediate cause of the 

 phenomenon ; and if we wish to know why the 

 solubility has diminished on the mutilated surface 

 Ostwald explains it to us by showing that crystal- 

 lization tends to form a polyhedron in which the 

 surface energy is a relative mimimum. 



