246 Johnston and Adams — High Pressures on the 



I. The metals crystallize as pure components from all their 

 melts. Examples studied: Zn— Cd, Cu — Ag. In this case 

 there is no essential difference between the conglomerates 

 obtained by fusion and compression. There are, however, 

 differences in structure; the former contains .a lamellar eutec- 

 tic, the latter is composed of irregular and irregularly distrib- 

 uted fragments of the components. 



II. The metals form compounds, but no mix-crystals. Exam- 

 ples studied : Mg with Pb, Sn, Zn, Sb, Bi. In these cases a 

 small but noticeable amount of the compound is found in the 

 block obtained by alternately compressing and filing down 

 again. In general the block is composed of fragments of the 

 components surrounded by films of the compound. 



III. The metals form one or more series of mix-crystals. 

 Examples studied : the compound MgCd with Mg and Cd ; 

 Bi-Tl, Pb-Tl, Sn-Cu, Zn-Cu, Al-Mg. In such cases the 

 condition of the compressed block varies with the lapse of 

 time after compression. Diffusion, promoted by the intimate 

 contact consequent upon the compression, sets in and proceeds 

 at a rate characteristic of the particular system ; this is usually 

 extremely small at ordinary temperatures, but in some cases is 

 noticeable in the course of a few days. As diffusion progresses 

 the condition of the compressed block approaches gradually 

 the equilibrium condition. The rate of diffusion increases very 

 rapidly at higher temperatures •* so that by keeping a com- 

 pressed block for a sufficient time at a temperature away 

 below its lowest melting point, its structure becomes identical 

 with that of the block obtained by fusion. 



It is to be noted, however, that these effects are not the 

 direct result of the compression, for a block immediately after 

 compression is for practical purposes entirely a conglomerate 

 of fragments of the two components. The sole effect of 

 compression in these cases is to effect intimate and thorough 

 contact between the two components ; by so doing, it promotes 

 diffusion, which is the primary cause of the changes observed. 

 Moreover, it is almost certain that the same state of equilibrium 

 would be reached by the action of diffusion, without previous 

 compression of the system, provided that sufficient time is 

 given — a condition easily fulfilled in geological work. In 

 those cases in which the metals form both compounds and 

 series of mix-crystals (as in some of the systems mentioned 

 under III) the total effect is an aggregate of the effects sum- 

 marized under II and III. 



Hence, the general conclusion of the whole matter is, that 

 compression alone does not result in the production of true 



* Thus, diffusion in the system Bi — Tl is more than 1000 times as great at 

 120° as at ordinary temperature. 



