172 Drs. Myers and Braun on the Decomposition 



ment, and by Griffiths, taking the value found in each case 

 at 15° as unity, the two curves would be practically identical. 

 This seems to show that the difference between the value of 

 the mechanical equivalent found by Rowland, and that found 

 by Griffiths and by Schuster and Gannon using electrical 

 methods, may not be due to an error in the experiment itself, 

 but to some error in the electrical standards of resistance, or 

 of electromotive force. 



These results are in almost absolute agreement with those 

 obtained by Mr. Waidner and Mr. Mallory. (See page 165.) 



XXVI. On the Decomposition of Silver Salts by Pressure. 

 By J. E. Myers, M.Sc, Ph.D., late 1851 Exhibition Science 

 Scholar, and F. Bkaun, Ph.D., Professor of Physics in the 

 University of Strassburg i.E.* 



AS was demonstrated by Carey Leaf some years ago, 

 silver salts and others may be decomposed by the 

 application of pressure. By pounding a portion of the salt 

 in a mortar one may readily effect decomposition. 



A convincing proof is obtained by compressing a halogen 

 salt of silver in the absence of sunlight and then subjecting 

 the compressed mass to the developing and fixing processes of 

 photography. A black residue of finely divided silver is the 

 result. Exposure of the salt to sunlight, before compression, 

 makes of course this test useless : the difficulty may be over- 

 come by employing a mixture of AgN0 3 and Kli (R=C1, 

 Br, or J) in the first instance. 



The decomposition is most marked in the case of bromide 

 of silver. 



Two different meials immersed in liquid bromine constitute 

 a galvanic element J. The couple Ag, Br, Pt has an E.M.P. 

 of about 0"95 volts. It appeared therefore of interest to 

 determine the E.M.F. developed when AgBr is compressed 

 between electrodes of platinum and silver. 



The salt was contained in a so-called diamond mortar, 

 which was placed under a press and subjected to a pressure 

 of 3000-5000 atmospheres. The apparatus did not admit of 

 a direct measurement of pressure, but for purposes of com- 

 parison, by using the same quantity of salt and turning the 

 lever to the same position the same pressure (approximately) 

 could be applied. The pestle was insulated from the mortar 

 by means of a thin sheet of mica bent into a cylindrical 

 * Communicated by the Physical Society: read June 11, 1897. 

 t Phil. Mag. xxxi.'p. 323 (1891) ; xxxiv. p. 4b* (1892; ; xxxvi. p 351 

 (1893j ; xxxvii. pp. 31 and 470 (1894). 



| F. BrauD, Wied. Ann. xvii. p. 610 (1862). 



