W. Spring — Compression of Powdered Solids. 289 



would transform one variety of sulphnr to the other at a tem- 

 perature of 18°. So it is not very astonishing that I arrived at 

 this result with 6,000 atm. pressure, and I acquired this fact 

 four years prior to the works of vant Hon and Reicher. 



Again, pressure intervenes as such to facilitate phenomena of 

 solid diffusion, when the volume of the matter after diffusion 

 is smaller than that before diffusion. This I have already 

 proved by numerous experiments, and will do so again in some 

 I have already had in hand for a long time. 



Mr. Hallock says on the subject of solid diffusion, " I think 



no one would expect that a piece of copper and tin 



soldered together would diffuse and form a homogeneous 

 bronze, nor would they expect that simple cubic static pressure 

 would promote these reactions." To my mind Mr. H.'s opin- 

 ion would be valid if the resulting bronze were less dense than 

 the mixture of copper and tin, but in the opposite case there 

 would be found, not a homogeneous bronze, but an interpene- 

 tration of the copper and tin at the surface of perfect contact, 

 which would cease when the difference in density of successive 

 infinitely thin layers, no longer had too high a value. This I 

 have proved for the reaction between Na 2 C0 3 and BaS0 4 — 

 and conversely — by studying the influence of time on the phe- 

 nomenon and by showing that after some weeks a state of equi- 

 librium is set up between the two bodies.* 



[ am aware that these results may appear extraordinary, but 

 I must express myself categorically, as they represent facts. I 

 will also add that on his side Mr. C. Roberts Austenf has- ob- 

 tained the blue alloy of copper and antimony at the surface of 

 contact of the metals under pressure. 



I have neither exhausted my reply nor the subject, but I will 

 let things remain in statu quo at present, as I think I have suffi- 

 ciently shown that I never attributed an active part to pressure 

 in the welding of bodies, but that it intervenes as such only 

 when a decrease in volume is necessary to produce a physical 

 or chemical change of state at a given temperature, and also 

 because I shall treat all those points in detail when uniting my 

 results in book-form, after the termination of my experiments. 



* Societe cliimique de Paris, 1884 et 1885. 

 f From a letter of Mr. Austen to the author. 



