528 M. C. Lea — Disruption of the Silver 



could be brought out by a reducing agent. The raised por- 

 tions of the embossed work exerted a stronger pressure on the 

 sensitive film than the rest of the card and these portions 

 darkened when acted upon by a reducing agent. In the same 

 way, the lines traced with a glass rod blackened under a devel- 

 oper. In each case, it was the portions which had been sub- 

 jected to pressure which yielded first to the reducer. It was 

 therefore clear that in the molecules which had received this 

 slight pressure the affinities of the atoms had been loosened. 



To bring these phenomena fully into line with the others it 

 is now necessary to prove that an increased pressure can take 

 the place of a reducing agent and disrupt the molecule. And 

 this is actually the case. 



It was found that the breaking up could be produced in two 

 ways, by simple pressure, and by shearing stress. Silver 

 chloride and bromide formed and washed in absence of active 

 light were subjected to these agencies. 



1. Simple Pressure. — In the first trial made with silver 

 chloride, it was enclosed in asbestos paper which had been first 

 ignited with a blast lamp to remove all traces of organic mat- 

 ter present. This method was tried in order that the chloride 

 should be in contact with perfectly inactive material only, but; 

 it was not found to answer. The great pressure employed 

 forced the dry chloride into the pores of the paper cementing 

 it together, so that the opposite sides could not be separated. 

 Platinum foil was then substituted with satisfactory results. 

 With a pressure of about one hundred thousand pounds to the 

 square inch, maintained for twenty-four hours, the chloride 

 was completely blackened except at the edges, where owing to 

 greater thinness the pressure was less. Very bright foil was 

 used in order to detect the slightest discoloration that might 

 occur, but none resulted ; it was impossible to distinguish the 

 portions which had been in contact with the darkened chloride 

 from those that had not. The chloride did not assume the 

 usual chocolate color, but changed to a deep, greenish black. 



Silver bromide gave exactly the same results. It should be 

 mentioned that the silver chloride and bromide were each pre- 

 cipitated with an excess of the corresponding acid. 



As silver iodide precipitated with excess of potassium iodide 

 is not darkened by light, it seemed improbable that it should be 

 by pressure. The experiment was however tried and it was 

 found that the iodide darkened fully to the same extent as the 

 others This result surprised me so much that the experiment 

 was repeated with every possible precaution. The result left 

 no doubt that silver iodide, as well as the chloride and brom- 

 ide is blackened by great pressure. All three silver haloids 

 take on the same coloration, an intense greenish black. It was 



