90 Correspondence — Major-General McMahon. 



as those of Cailletet, Pfafif and Spring," refers to one of the experi- 

 ments of the latter in the course of which sulphur and copper filings 

 subjected to a pressure of 5000 atmospheres were converted into 

 crystallized copper sulphide. Mr. Harker comments on this as 

 follows : — " So much of the mechanically-developed energy as takes 

 the form of heat is carefully removed ; but chemical combination 

 still takes place. It follows that the energy absorbed in this com- 

 bination comes directly from the mechanical work done, without 

 the intervention of heat." 



I have no desire to interpose in the controversy between Mr. 

 Harker and Dr. Irving, but, without doing this, I may point out 

 that the explanation offered by Mr. Harker does not appear to be 

 in accord with that offered by M. Spring himself. 



In the American Journ. Science, xxxvi. (1888) pp. 286-289, Spring 

 remarks regarding his " researches on the compression of powders," 

 '' To my mind pressure was not an active agent in the matter ; but 

 only the means to the end, and I looked for the effects to contact 

 alone. ... In another place [Bull. Soc. Chemique, 1884], I said 

 with regard to chemical action produced by my experiments — 

 ' one must not lose sight of the fact that pressure is not a chemical 

 agent to the same extent 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 compression. I now see that I was unwise in thus wishing to 

 economise 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 the failures in pre- 

 liminary experiments made under the ordinary conditions ;" and M. 

 Spring goes on to explain that when he did not operate in vacuo the 

 presence of air between the particles hindered intimate contact 

 between them and thus prevented chemical action, pressure not- 

 withstanding. 



In a previous communication to the American Journ. Science 

 (vol. XXXV. 1888, p. 78) Mr. Spring wrote : " Since then [1880] 

 new experiments, still in part unpublished, have made me recognize 

 the importance of the part that a certain degree of temperature plays 

 in these phenomena, so that for the solid state, as well as for the 

 gaseous one, a critical temperature would be remarked, above or 

 below which the changes of simple pressure would be no longer 

 possible." 



Spring, therefore, in the conclusions arrived at as recently as the 

 close of 1888, attributes the chemical action set up in his experiments 

 to contact plus a certain degree of temperature. Pressure is merely 

 the matrimonial agent, so to speak, that brings the highly susceptible 

 particles together ; but it is to contact plus heat that, according to 

 Spring, the chemical action is due. 



C. A. MoMahon. 



20, Nbvern Square, \2th Jan. 1891. 



