S. Tctber — Growth of Crystals. 555 



less than the volume of the water alone.* All the salts used 

 in the experiments described in this paper go into solution 

 with contraction in volume. The writer has tried to obtain 

 some evidence of pressure during the crystallization of 

 ammonium chloride, one of the few salts that dissolve in water 

 with an expansion in the volume of the solution, but so far has 

 obtained only negative results. This question, however, is not 

 regarded as settled, and the experiments are being continued. 



Pressure tends to increase the solubility of a salt that goes 

 into solution with a contraction in the total volume. The 

 separation of such a salt from solution must be accompanied 

 by a corresponding increase in volume, and if the separation 

 takes place in a closed and limited space, the pressure developed 

 may be enormous, probably many times the crushing strength 

 of the substance. 



If a crystal or crystalline mass should be so enclosed by 

 other matter that the material for growth reached it only by 

 diffusion through subcapillary openings, then the pressure 

 developed during crystallization might be very largely due to 

 the accompanying increase in volume. If this hypothesis 

 proves to be true it may help to explain the variation in the 

 character of the concretions found in rocks that differ in 

 texture. Those concretions, that largely exclude the material 

 composing the rock in which the} 7 grow and in so doing push 

 apart the planes of lamination, seem to be limited chiefly, if 

 not entirely, to the iine-grained rocks, such as shales and slates, 

 in which the openings are for the most part subcapillary in 

 size. On the other hand there are many concretions that are 

 relatively impure, because they include much of the surround- 

 ing material, and these do not show any evidence of having 

 exerted pressure on the inclosing rock, since the bedding planes 

 of the latter may often be traced through the concretions with- 

 out break in their continuity. 



This class of concretions seems to be limited largely if not 

 entirely to sandstones and other coarser-textured rocks in 

 which the openings are for the most part capillary and super- 

 capillary in size. In extreme cases the concentrated matter 

 merely fills the interstices and cements loose-textured material 

 together to form indistinct nodular masses. 



Highly pure siliceous concretions and pyrite crystals may 

 develop in limestones and similar calcareous rocks without dis- 

 torting the enclosing material, for such rocks are relatively 



*Thomsen, Therm. Untersuch., I, 45, 1882. MacGregor, Trans. Rov. Soe. 

 Can. 1890, 19; 1891,15. Trans. Nova Scotia Inst. Sci., vii, 368^ 1890. 

 Traube, Zs. anorg. Chem., iii, 1, 1892. These references cited in G. P. 

 Baxter's "Changes in volume upon solution in water of the salts of the 

 alkalis," Jour. Am. Chem. Soc, vol. xxxiii, p. 922, 1911. 



