252 Johnston and Adams — High Pressures on the 



density of the substance, and on the temperature of experi- 

 ment ; consequently there is a rough parallelism between ease 

 of flow and melting point, the substances with the highest 

 melting points being those which at any fixed temperature 

 require the greatest force to weld them. 



(2) That compression will not in general cause crystalliza- 

 tion of a material, or the transformation of one form into 

 another ; it will do so in certain cases, where the other con- 

 ditions are such that the velocity of transformation is appreci- 

 able; and will cause a partial transformation wherever the 

 conditions are such that a partial melting occurs. 



(3) The conditions for the furtherance of chemical reactions 

 by compression are similar. With uniform pressure the reac- 

 tion takes place only at the surfaces of contact, and hence only 

 a small total amount of compound is formed ; but when the 

 compression is not uniform, there is a renewal of the surfaces 

 which allows the reaction to proceed farther. This renewal is 

 the result of a direct abrasive action and may also be advanced 

 by a melting, and flowing away, of the reaction product, if its 

 melting pressure is attained. In other words, the effect of 

 non-uniform compression is precisely the same as that of grind- 

 ing together the reacting substances in a mortar. 



(4) Several of the criteria used by Spring to determine the 

 condition of compressed systems are altogether insufficient; 

 for example, no statement whatever can at present be made as 

 to the equilibrium in the solid state of the system 



BaS0 4 + Na 2 C0 3 ^z± Na 2 S0 4 + BaC0 3 



The only justifiable deduction possible from his work with this 

 system is that, on adding water to a mixture composed of 

 either of the above pairs of substances, the amount transformed 

 in a given short time depends upon the thoroughness with 

 which the mixing was performed — a conclusion by no means 

 new. 



(5) "With regard to the formation of alloys, the conditions 

 are the same as those affecting chemical reaction. Where 

 solid solutions are possible — either between metals or other 

 substances — the intimate contact produced by compression 

 promotes diffusion ; but it is altogether probable that the 

 effect of compression in furthering the formation of solid 

 solutions (and possibly in promoting diffusion) is limited to this 

 subsidiary role. 



In general then, the effectiveness of compression applied to 

 solid systems depends (1) upon its character (uniform or other- 

 wise), (2) upon certain physical constants of the material, nota- 

 bly its melting point, but also its latent heat of melting (the 

 value of which is known at present for a very few substances 



