288 W. Spring — Compression of Powdered Solids. 



what I said on this point in 1880,* — " so that in my prelim- 

 inary experiments I had not operated in vacuo and must conse- 

 quently have enclosed a notable quantity of air in the powders 

 I compressed ; this, besides preventing by its presence intimate 

 contact between the particles of solid matter and hindering 

 their union, etc." 



If a different pressure is necessary for different bodies, it is 

 solely because all bodies are not equally hard and not because 

 pressure is the immediate cause of their welding. 



Better yet, in all articles I have hitherto published on the 

 chemical reactions of powdered solids I have shown the in- 

 fluence of time on the chemical phenomena, by leaving the 

 compressed bodies undisturbed for more or less time, and out 

 of the compressor. Here it is evident that pressure had nothing 

 more to do, as it existed no longer. Quite recently I sent an 

 article to the Zeitschrift fur Physik u. Chemie, containing 

 the results of experiments of this kind, which lasted four years, 

 and showing how a chemical reaction can complete itself if the 

 matter is allowed the time for diffusion. They also explain 

 Mr. Hallock's results on the formation of alloys. 



Finally, from a rational point of view, there is no reason for 

 assigning an active part to pressure in the phenomenon of weld- 

 ing of bodies pure and simple, for the act of welding by no 

 means brings about a permanent decrease in the volume of the 

 matter, and from that moment it is not conceivable how it' 

 would possibly act otherwise than to establish as perfect a state 

 of contact as possible. When Mr. H. states that welding takes 

 place better by " motion under pressure" he. is quite correct, 

 for then the sliding of the grains on one another is eminently 

 adapted to bring about perfect contact, because it expels inter- 

 posed air and moisture, exactly as when clay is kneaded with 

 the fingers. 



We will now consider another view of the question, and ex- 

 amine the phenomena in which pressure acts as such, i. e. by 

 the diminution in volume it causes in the matter operated on. 



In my experiments published in 1880 I found that pressure 

 brings about changes of allotropic state in bodies, when the 

 least dense variety is compressed at a certain temperature. This 

 is what pressure does. Many proofs now exist in favor of my 

 conclusions. I will confine myself to quoting the fine re- 

 searches of van't Hoff and Keicher on the shifting of the 

 transition point by pressure. t These works have proved, in 

 theory and practice, that rhombic sulphur passes to the mono- 

 symmetric variety at 95*6° at the ordinary pressure, and at 

 96*2° under 12 atm. ; in other words 12 atm. pressure shift 

 the point of transition 0*6°. This being the case 1,552 atm. 



* Loc. cit. f Etudes de dynamique chimique, Amsterdam, 1884, p. 198. 



