50 Disruption of the Silver Haloid Molecule. 



of heat thus generated has anything to do with the reaction. 

 The heat is not perceptible, it is momentary ; and it has been 

 elsewhere shown that though moist silver chloride can be 

 broken up by heat, the action is slow even at a temperature 

 of 100° C. 



In the case of simple pressure, heat certainly plays no part. 

 The material is small in quantity, is folded up in metal, is 

 placed between large and heavy pieces of metal, and the pres- 

 sure is applied gradually by means of a screw. Even sup- 

 posing a slight increase of temperature, it could not exceed 

 one or two degrees and would be momentary. As just 

 remarked, heat does not produce an effect except at about 

 100° C. and after many hours. 



The powerful affinity which exists between silver and the 

 halogens is well known. That this affinity can be counteracted 

 and annulled by simple pressure — that the halogen can in part 

 be forced out of the molecule by mechanical means unaided 

 by heat — is remarkable. 



It need scarcely be said that this phenomenon has nothing 

 in common with decompositions produced by mechanical force 

 in substances such as silver or mercury fulminate, nitrogen 

 chloride, and similar explosives. Such substances are all 

 formed by endothermic reactions, and their decompositions 

 are exothermic. Heat does not need to be supplied, but only 

 what Berthelot has named a " travail prSliminaire" an impulse 

 to start the reaction. But silver haloids are formed by exo- 

 thermic reactions ; consequently their decompositions are 

 endothermic, and require that the energy which was disen- 

 gaged in their formation should be returned to effect their 

 decomposition. The experiments described in this paper show 

 that mechanical force may be made to supply this energy, 

 and so play the part of light, electricity, or heat, without 

 previous conversion into any other form of energy. 



The thermochemical reactions of the silver haloids have 

 been studied by Berthelot, and their reductions were found 

 to be endothermic*. There can be no doubt, therefore, that 

 an endothermic reaction can be brought about by simple 

 pressure. 



Philadelphia, April 1892. 



* Micanique Chimique, vol. ii. p. 411. The reduction of silver chloride 

 to metal involves an absorption of cal. 29-4. That to hemichloride has 

 not been measured, but is, according to Berthelot, also endothermic. See 

 also Ditte, Les Metaux, i. pp. 232, 233. 



