21. C. Lea — Endothermic Deco?nposilions by Pressure. 415 



(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 var- 

 nish of silver photochloride, thus indicating a partial reduction 

 to subchloride. It has since proved that there is no more effec- 

 tual method than this of applying shearing stress and that in 

 this wa} T a number of quite stable chemical compounds 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 exer- 

 tion of great pressure on the pestle. It is also absolutely essen- 

 tial 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 35H turns in 10 inches respectively. The mechanical efficiency of such a 

 screw is that of one having 320 turns to the inch if such a thing were practica- 

 ble, at the same time that a thoroughly strong construction can be obtained. 

 The massive steel nut to advance \ of an iuch would require 40 full turns of this 

 screw. 



This arrangement compares as follows with that previously employed. In it to 

 cause the vis^ jaws to approach by 1 inch required that the point of the lever at 

 which the force was applied should pass through a space of 113 1 feet: this rela- 

 tion. 1 inch to 1131 feet cr 1 : 13o7-2 gives the measure of the efficiency of the 

 instrument. 



With the double screw, on the ether 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 \ of a mile. The circumference described by the 

 lever being approximately 1 L'-j feet and the screw requiring 40 turns to advance 

 the nut \ of an inch we get the proportion of 1 inch to 4,000 feei, or 1 to 48,000. 

 which is the measure of the efficiency of such an instrument. Therefore, suppos- 

 ing two men to pull on the end of the lever each with a pull of 100 lbs. the pres- 

 sure exerted on the nut (disregarding loss by friction) would be 9,600,000 lbs., 

 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 the 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 transforming mechanical into 

 chemical energy. 



