W. Spring — Compression of Powdered Solids. 287 



I regret to be obliged to state that this new objection of Mr. 

 Hallock's falls wide of its mark, like the first ; this time it is a 

 case of misunderstanding fortified by confusion, for he again 

 repeats exactly what I have always asserted myself. I may, 

 therefore, be allowed to explain in English this time, on account 

 of the importance of the subject, and to avoid a recurrence of 

 misinterpretations, which simply mean a loss of time and labor 

 to everyone concerned. 



When ten years ago* I began my researches on the compres- 

 sion of powders my sole object was to ascertain whether solid 

 bodies possessed the property of welding when their particles are 

 put in perfect contact. I desired to find out whether the phe- 

 nomenon of regelatibn could be generalized, since Faraday had 

 shown that contact, or an extremely slight pressure, suffices for 

 its production, contrary to J. Thomson's view, who said pres- 

 sure was the cause of the phenomenon. To realize this contact 

 I thought it would be sufficient to press the bodies strongly to- 

 gether. To my mind, moreover, pressure was not an active 

 agent in the matter, but only a means to the end, and I looked 

 for the effects to contact alone. I have expressed myself thus 

 on this point in nearly all my articles and it will therefore be 

 sufficient to quote the following. 



After having recalledf the fact that certain bodies, such as 

 sodium nitrate, become a coherent mass when their powders 

 are left undisturbed in a bottle for a certain space of time, and 

 that the coherence of the mass depended upon contact, I said, 

 " now, to increase the number of points of contact in a pow- 

 dered body, it will be sufficient to put it under pressure heavy 

 enough to cause the spaces between the fragments of the body 

 to become filled up with their debris." 



In another placed I said with regard to chemical reaction 

 produced in my experiments — " one must not not lose sight of 

 the fact that pressure is not a chemical agent to the same ex- 

 tent as heat or electricity." But as I have always thought that 

 contact was brought about by compression I have often for the 

 sake of brevity, spoken of " welding due to pressure" instead of 

 always saying " welding due to contact produced by compres- 

 sion" I now see that I was unwise in thus wishing to econo- 

 mize my time. 



Besides, as conclusive proof that it is always to contact that 

 I assigned welding phenomena, chemical reactions and also in 

 part the diffusion of solids, there is the fact that I deemed it 

 necessary to operate in vacuo, on account of failures in prelim- 

 inary experiments made under the ordinary conditions. This is 



* Bulletin de l'Academie de Belgique, II, xlvi, 1878. 



f Bulletin de l'Academie de Belgique, II, xlix, p. 336, 1880. 



% Bulletin de la Societe chimique de Paris, vol. xli, p. 497, 1884. 



