240 Johnston and Adams — High Pressures on the 



tion by abrading the surfaces and bringing new surfaces into 

 contact. There is also reason to doubt Spring's statement that 

 some of the reactions had gone to completion ; he applied no 

 decisive chemical tests, but based his conclusion solely on the 

 general appearance of the product (or of its fracture) when 

 examined with the naked eye or microscopically — a criterion 

 which cannot be regarded as very satisfactory for judging the 

 extent to which a reaction has proceeded. 



The general conclusion from this work is, therefore, that 

 under the particular conditions of experiment combination 

 between certain substances takes place to some extent ; this, 

 however, is practically a restatement of what has long been 

 known, namely that grinding in a mortar will occasionally 

 induce some degree of chemical action between solids. But 

 we are in no wise justified in making the general statement 

 that chemical action is promoted by compression, or a fortiori 

 by uniform pressure ; all that we may say is, that certain reac- 

 tions are furthered by the close contact and grinding action 

 produced by, and during, the application of compression ; as 

 soon as the compression has reached a steady value, action 

 ceases except for the very slow diffusion which appears to be 

 possible with certain systems and to be promoted by the inti- 

 mate contact consequent on compression.* 



From the standpoint adopted in this paper we should expect 

 that the reactions most readily produced by compression would 

 be those in which the melting point of the product is most 

 easily depressed by the action of pressure. For the reaction 

 will take place at the surfaces of contact but can proceed only 

 so long as fresh surfaces continue to be exposed by removal of 

 the thin layers of the reaction product formed at the contacts ; 

 this removal may be effected either by a melting, by direct 

 mechanical abrasion, or by both combined. The evidence is 

 at present too meager to admit of a demonstration of a parallel- 

 ism between the extent of the reaction, and the properties of 

 the reaction product ; but at least it in no wise contradicts 

 this idea. 



B. The Alleged Reaction Between Solid Salts, 



1. The Equilibrium : BaS0 4 + Na 2 C0 3 ^—> BaC0 3 + NaS0 4 . 



The effect of compression in promoting the above reaction 

 between the solid substance has probably been quoted more 

 often than any other part of Spring's work ; and the results as 

 stated by Spring (and quoted and re-quoted by others) have 

 been used to support many geological speculations. The per- 

 missible conclusions from these results, however, differ con- 



* Of. p. 248. 



