216 Johnston and Adams — High Pressures on the 



solubility,* and renders the solutions supersaturated as soon as 

 they are out of contact with the compressed solid. This is merely 

 another statement of the fact that on straining a crystal its solu- 

 bility on the strained face is increased ; consequently a strained 

 crystal in contact with a saturated solution in any solvent dis- 

 solves on the strained faces, and is redeposited where there is 

 no strain. The effect of this is that the crystal changes in such 

 a way as to diminish the strain on it — an example of the well- 

 known principle that the readjustment of a system following 

 disturbance of the equilibrium is always such as to minimize 

 the effect of the disturbing factor. 



Le Chatelier accounts in this way for the solidification of 

 natural beds of rock-salt, gypsum, calcium carbonate, and clay. 

 To test this matter directly he compressed sodium chloride or 

 sodium nitrate in contact with its saturated solution to about 

 200 atmospheres for a period of eight days, and found that 

 blocks resembling rock-salt and marble were formed in this 

 way. Somewhat earlier than this, Spring f had made analo- 

 gous experiments, compressing to about 6000 atm. a great vari- 

 ety of substances, previously powdered and moistened with a 

 few drops of water. He found that the metals would not weld 

 together until the water was squeezed out ; with most other 

 substances compact blocks were obtained. Even with very 

 slightly soluble substances — e. g., calcium carbonate — the effect 

 of the water was noticeable, as the blocks obtained in this way 

 from the moistened powder were more compact than those 

 resulting from compression of the dry powder. 



]STow, uniform pressure (acting on both solid and liquid), 

 increases the solubility of those substances the dissolution of 

 which is attended with & decrease oi total volume; consequently 

 uniform pressure is sufficient to account for the consolidation 

 of substances belonging to this category. But uniform pres- 

 sure will not account for the consolidation of those substances 

 the dissolution of which is attended with an increase of vol- 

 ume ; whose solubility, therefore, diminishes with increase of 

 pressure. Unequal pressure (acting in excess of the solid phase) 

 is competent to account for the consolidation in all cases ex- 

 cepting one relatively infrequent case: namely, with substances 

 belonging to the second category, when AP is small compared 

 to P (see footnote, p. 213). This case apparently occurred with 

 two substances tried by Spring, for he states that ammonium 

 chloride and potassium iodide gave friable blocks, much softer 

 than those obtained by compressing the dry powders. Spring 



*The amount of this increase of solubility can be computed by equations 

 in every respect analogous to those applicable to the effect of pressure on 

 the melting point. 



fZs. phys. Chem.,i, 532-5, 1888; Ann. soc. geolog. Belg., xv., 156, 1888. 



