Decompositions obtained by Pressure. 33 



I. 



It was mentioned in a previous paper on the decomposition 

 of the silver haloids by mechanical force that when silver 

 chloride was sharply ground for some time in a mortar, both 

 the pestle and mortar became covered with a deep purple 

 varnish of silver photochloride, thus indicating a partial re- 

 duction to subchloride. It has since proved that there is no 

 more effectual method than this of applying shearing-stress, 

 and that in this way a number of quite stable chemical com- 

 pounds formed by exothermic reactions can be broken up. 

 The mortar and pestle should be very solid and of unglazed 

 porcelain. With metal there would be danger of action 

 between the metal and the substance, and with agate mortars 

 sufficient force cannot well be applied. In many cases success 

 depends on the exertion of great pressure on the pestle. It 

 is also absolutely essential that the quantity of material acted 

 upon should be small. When a larger quantity is employed 

 the particles slip or roll over each other and thus escape the 

 action of the stress. It is no doubt for this reason that the 

 very remarkable results which can be obtained in this way 

 have hitherto escaped attention. 



A small quantity, a few decigrams, of the substance having 

 been placed in the mortar, the first thing is to spread it' in a 

 thin uniform coat over the bottom and part of the sides. The 

 pestle is then to be rotated with the utmost force that can be 

 exerted. 



Sodium Chloraurate. — The salts of gold are particularly 

 well adapted to this examination, as the reduction is complete 

 and the gold appears in the metallic state so that it can be 

 weighed and the exact amount of reduction can be fixed. It 



of 113-1 feet : this relation, 1 inch to 113-1 feet or 1 : 1357*2, gives the 

 measure of the efficiency of the instrument. 



With the double screw, on the other hand, to cause the nut to advance 

 § of an inch, the end of the lever (2 feet long) must pass through a space 

 of 500 feet, or in the proportion of 1 inch to over f of a mile. The circum- 

 ference described by the lever being approximately 12-| feet, and the screw 

 requiring 40 turns to advance the nut ^ of an inch, we get the proportion 

 of 1 inch to 4000 feet, or 1 to 48,000, which is the measure of the 

 efficiency of such an instrument. Therefore, supposing two men to pull 

 on the end of the lever each with a pull of 100 lb., the pressure exerted 

 on the nut (disregarding loss by friction) would be 9,600,000 lb., which 

 could be doubled by using a 4-foot lever. 



Such a combination is quite practicable, the only real difficulty being to 

 obtain sufficient solidity of construction to resist strain. I had made 

 drawings for this instrument, but gave it up in consequence of observing 

 the enormously greater efficiency of shearing-stress as a means of trans- 

 forming mechanical into chemical energy. 



Phil Mag. S. 5. Vol. 37. No. 224. Jan. 1894. D 



