48 Mr. M. Carey Lea on the Disruption of the 



not assume the usual chocolate colour, 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 

 precipitated 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 bromide, is blackened by great pressure. All three silver 

 haloids take on the same coloration — an intense greenish 

 black. It was found best to use the material air-dried. If 

 at all moist the platinum foil bursts under the pressure and 

 the experiment is invalidated. The air-dried salt retains a 

 sufficient quantity of moisture. 



2. Shearing -Stress. — As a means of applying this form of 

 force, the silver chloride, precipitated with excess of hydro- 

 chloric acid and well washed, was put into a porcelain mortar 

 and well triturated. The improbability that the small quan- 

 tity of force that can be applied in this way would break up a 

 stable molecule like that of silver chloride seemed so great, 

 that at first a substance tending to aid the reaction was added. 

 Tannin was selected, and when forcibly ground up with silver 

 chloride the latter was soon darkened. Next a substance 

 capable of taking up acid, but having no reducing action, was 

 tried. Sodium carbonate was used. This also caused the 

 chloride to darken. Finally, it was determined to ascertain 

 if the molecule of silver chloride could not be disrupted by 

 stress alone. The chloride was placed in a chemically clean 

 porcelain mortar and well triturated. For some time no 

 effect was visible. After about ten minutes' action dark 

 streaks began to appear, and after five minutes' more work a 

 considerable portion of the chloride was darkened. The end of 

 the pestle was covered with a shining purple varnish. It had 

 not become perceptibly warmer to the touch. On the violet- 

 purple substance nitric acid had no action, but aqua regia slowly 

 whitened it. It was therefore what I have proposed to call 

 silver photochloride, that is, a molecular combination of chlo- 

 ride and hemi chloride. This experiment was carefully repeated 

 with the same result. Silver bromide similarly treated gave 

 a similar result. It was noticed that both chloride and bro- 

 mide, in darkening, took on the familiar colour between 

 chocolate and purple, so generally seen in the darkening of 



