Chemistry and Physics. 531 



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 chlo- 

 ride and similar explosives. Such substances are all formed by 

 endothermic reactions and their decompositions are exother- 

 mic. Heat does not need to be supplied, but only what Ber- 

 thelot has named a " travail preliminaire," an impulse to start 

 the reaction. But silver haloids are formed by exothermic 

 reactions, consequently their decompositions are endothermic 

 and require that the energy which was disengaged 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. 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. On the Polarization of Platinum by Oxygen and Hydrogen. 

 — A series of measurements has been made by Markofsky to ascer- 

 tain the difference of potential developed between a plate of plati- 

 num entirely immersed in dilute sulphuric acid, freed from air by 

 boiling, and a second plate partly immersed in this liquid and 

 partly exposed to an atmosphere of pure hydrogen or pure oxy- 

 gen gas. The experiments were varied by using an acid contain- 

 ing platinum sulphate, the strength of the solution being different 

 in different experiments. Carbon electrodes were also employed 

 in place of platinum. He finds that the potential-difference 

 between a platinum plate in hydrogen and one in dilute acid free 

 from gas, is 0*646 volt. If oxygen be substituted for the hydro- 

 gen, the polarization is reversed in direction and is equal to 

 0*372 volt. Moreover, this potential-difference remains the same 

 whether the gases employed are produced electrolytically or by 

 chemical processes. The author observed that the addition of 

 platinum sulphate to the acid increases the potential-difference in 



*Mechanique chimique, vol. ii, p. 411. The reduction of silver chloride to 

 metal involves an absorption of Cal. 29*4. That to hemichlonde has not been 

 measured, but is, according to Berthelot, also endothermic. See also, Ditte, Les 

 Metaux, I, pp. 232, 233. 



