Haloid Molecule by Mechanical Force. 529 



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 quantity 

 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 capa- 

 ble 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 chloride and hemichloride. 

 This experiment was carefully repeated with the same result. 

 Silver bromide similarly treated gave a similar result. It was 

 noticed that both chloride and bromide in darkening took on 

 the familiar color between chocolate and purple, so generally 

 seen in the darkening of these silver salts and differing strik- 

 ingly from the greenish black color assumed by all three silver 

 haloids under simple pressure. 



The fact that the platinum foil remained absolutely unat- 

 tacked when the silver haloid was reduced by simple pressure 

 in actual contact with it, is interesting and would seem to show 

 that in the reduction of the silver haloid the halogen is not at 

 any time set free but that water, if present, is decomposed at 

 the same moment with formation of halogen acid. 



The observations recorded in this paper prove the existence 

 of a perfect uniformity in the action of all kinds of energy on 

 the silver haloids. The balance of the molecule is at once 

 affected by the influence of any form of energy. A slight appli- 

 cation produces an effect which, though invisible to the eye, is 

 instantly made evident by the application of a reducing agent. 

 The bonds which unite the atoms have evidently been in some 



