of Silver Salts by Pressure. 



173 



form. A piece of platinum foil beneath the salt, and a short 

 cylinder of silver immediately beneath the pestle, formed the 

 electrodes, which were connected to the quadrants of an 

 electrometer. 



The following serves to give a general idea of the run of an 

 experiment : — 



(One Daniell element gave a deflexion of 132*1 scale- 

 divisions.) 



On applying pressure, immediate deflexion 

 after 2 minutes 

 ., 10 



15 



40 

 55 



85 



95 



105 



Scale-divisions. 



. 1-2 



. 2-8 



. 6-8 



. 72 



. 9-2 



. 9-0 



. 8-7 



. 8-2 



. 7-0 



. 5-0 



Pressure removed 



after which the deflexion rapidly sunk to zero. 



Thus the maximum E.M.F generated was about 0*07 

 Daniell. The direction of current was from Ag to Pt through 

 the compressed substance. 



A higher applied pressure produced an E.M.F. of 0'09 

 Daniell. 



The silver bromide was prepared from pure AgN0 3 by 

 precipitation with pure KBr. Before each experiment the 

 salt was carefully dried by heating for two hours or more at 

 a temperature of 150° C, and then allowed to cool in a 

 desiccator. Pure AgN0 3 showed no appreciable E.M.F. on 

 compression, neither did KBr : a mixture of the two in 

 equivalent quantities compressed between electrodes of Pt 

 and Ag had an E.M.F. of 0"08 Daniell. 



With AgCl and Agl the E.M.F.'s were smaller. 



Ag, AgCl, Pt = 0-03 Daniell. 

 Ag, Agl, Pt = 0-04 „ 



In both cases a marked darkening in colour was noticeable. 



Somewhat remarkable is the fact that under high pressure 

 AgBr becomes gradually transparent even when 3-4 millim. 

 thick. At the same time the electrical resistance increases 

 considerably. For instance, in one case the resistance 

 increased gradually from 3*65 . 10 3 ohms to 1462 . 10 3 ohms 

 (measured with alternating current and telephone) in the 

 course of 48 hours, pressure being continuously applied. 



Strassburg, Physik. Listi.tut, 



