Physical and Chemical Behavior of Solids. 237 



pressure which we have been able to employ in this laboratory 

 - — and we have used pressures up to 15,000 atm.* — will convert 

 calcite (sp. gr. — 2*71) into arragonite (sp. gr.=2*93) nor mar- 

 casite (sp. gr.=4*9) into pyrite (sp. gr.=5*0) ; nor did the latter 

 change take place to an appreciable extent at 425° under 2000 

 atm. pressure, although under ordinary pressure it does take 

 place about 450°. t These cases are cited merely to show the 

 folly of the statement that pressure will always actually pro- 

 duce the denser form. All that we may say is that, when the 

 reaction is reversible (enantiotropic changes) pressure tends to 

 displace the equilibrium in favor of the denser form, the actual 

 appearance of which depends mainly on the velocity of reac- 

 tion under the conditions involved ; when the reaction is 

 irreversible (monotropic changes, such as the two cases cited 

 immediately above appear to be) we can make no positive 

 statement as to the effect of pressure on the reaction, and can 

 determine this effect for each particular monotropic change 

 only by actual trial. The general statement may be hazarded 

 that the influence of pressure on monotropic changes will be 

 slight, with the important reservation that changes which we 

 are accustomed to regard as monotropic may cease to be so and 

 become enantiotropic when the system is exposed to high 

 pressure. Apart from this, pressure can in all probability 

 be effective only in changing the velocity of reaction, and, as 

 we have seen, it is likely that this effect is small. 



Effect of Compression in Promoting Chemical Reaction 

 Between Solids. 



Spring, in his first paper,;}: describes certain experiments on 

 the occurrence of chemical action through the agency of pres- 

 sure. Thus he states that copper and sulphur compressed to 

 5000 atmospheres react completely with the formation of black 

 crystalline cuprous sulphide ; that mercuric chloride and copper 

 filings react under 5000 atmospheres to form cuprous chloride 

 and mercury ; that the mixture of mercuric chloride and 

 potassium iodide, which is originally white, becomes red 

 under 2000 atmospheres, all traces of the colorless salt having 

 disappeared. On the other hand, nothing happened on com- 

 pressing mixtures of potassium iodide and mercuric sulphide, 

 mercury oxide and sulphur, or tartaric acid and sodium car- 

 bonate ; but then these are reactions which would be accom- 



* This pressure is equivalent to a depth of about 34 miles below the sur- 

 face of the earth, if we assume that the pressure at that depth is the weight 

 of a column of rock the average density of which is 2 "8. 



f Cf. further the work of G. Spezia, referred to later (p. 247). 



% Bull. Acad. Eoy. Belg. (2), xlix, 323-79, 1880. An earlier paper, pub- 

 lished in 1878, is merely a preliminary note. 



Am. Jour. Sci.— Fourth Series, Vol. XXXV, No. 207.— March, 1913. 



17 



