Physical and Chemical Behavior of Solids. 233 



the solid particles, of foreign liquid or gaseous material which 

 prevents the complete agglutination of the powder. Conse- 

 quently, it can be produced at will by compressing in the 

 presence of some gas or by having foreign matter on the 

 particles.* 



There remained four substances— bismuth, lead sulphide, 

 zinc sulphide, and mercuric iodide— for which Spring's results 

 conflicted with those of Jannettaz. In order to convince his 

 opponents, Spring exhibited his specimens, and performed new 

 experiments, in presence of Friedel and others, whereupon 

 Friedel writes in part as follows :f 



" The contradiction is more in the manner of expressing the 

 results than in the results themselves, which are altogether 

 comparable [with those of Friedel and of Jannettaz]. Never- 

 theless in certain cases in which Spring sees a crystalline condi- 

 tion, and where indeed evidences of crystallization may be de- 

 tected with a microscope, I cannot admit that there has been 

 crystallization as a direct consequence of the compression ; that 

 is to say, 1 cannot admit that several crystalline grains of the 

 original precipitate have welded togethrr so as to form a single 

 crystal, traversed by continuous cleavage planes ; une would 

 then have a single visible crystal and not the structure actually 

 observed, which can be denoted only by the term aggregate or 

 macrocrystalline structure, the crystalline grains being no larger 

 than those in the original powder." 



In only one case — metallic bismuth, which was squeezed out 

 through the space between plunger and bomb — will Friedel 

 admit a possible crystallization ; the evidence for it rests only on 

 the appearance of broken surfaces of the metal, and can hardly 

 be regarded as showing that a process of crystallization has 

 actually taken place as a result of the compression. In accord- 

 ance with this, Friedel concludes that there was no marked 

 crystallization by pressure under the conditions of Spring's 

 experiment, though he does not doubt that it may occur in 

 certain cases. 



This conclusion is in harmony with present experience. 

 Thus a substance — e. g. sodium chloride — which crystallizes in 

 the regular system may be compressed from powder into a 

 hard and fairly transparent block ; yet microscopic examination 

 of such a block, shortly after it had been formed by compres- 

 sion, showed that the original grains were merely stuck 

 together — that an agglomeration, but no formation of larger 

 crystals had occurred. Whether diffusion could in time pro- 



*Tlie presence of water will in general not produce schistosity, for the 

 reason that it dissolves to some extent the surfaces of the individual grains, 

 and in this way tends to behave as a cementing agent. See p. 216. 



f Bull. soc. chim. (2), xl, 526-8, 1883. 



