352 Mr. M. Carey Lea on Endothermic Reactions 



between substances whose tendency to combine was restrained 

 by their being in the solid form. This obstacle was removed by 

 subjecting them to great pressure, and the same remark applies 

 to some of the interesting experiments of Dr. Hallock. But 

 the reduction of the silver haloids and other reactions presently 

 to be described involve a quite different principle. The re- 

 actions produced are all endothermic ; energy is consumed in 

 accomplishing them, and this energy is supplied by mechanical 

 force. 



The combination of screw and lever affords the best means 

 of applying pressure. A careful study as to the most suitable 

 method of using these powers led to the selection of the 

 vice form. It was found, however, that the manufacturers of 

 heavy vices were unwilling to undertake to furnish vices with 

 jaws that would sustain the force intended to be exerted on 

 them, namely, that of a steel lever three feet long acting on a 

 screw with six turns to the inch. I was therefore obliged to 

 have them made under my own supervision. From a bar of 

 tough rolled iron 4 inches wide by 1^ inch thick, pieces about 

 18 inches long were cut and were forged into shape by a black- 

 smith ; where the jaws met they were faced with steel welded 

 on. Suitable screws were easily obtained, but the nuts fur- 

 nished with them having commonly a length of only an inch 

 would subject the thread to the danger of stripping. They 

 were replaced with nuts 4 inches in length, thus distributing 

 the strain on 24 turns of the thread. Two vices made in this 

 way over a year ago have endured severe use without the least 

 sign of strain. 



With a lever three feet in length between the centre of 

 the screw-head and the end at which the force is applied 

 acting on a screw with six turns to the inch, the multiplication 

 of force is 1357 times. As it is easy to apply a pull of a 

 hundred pounds or even much more, a pressure of 135,000 lb. 

 is easily obtained. As it was intended to keep the substances 

 which were to be subjected to pressure from any contact that 

 might affect them, they were folded up in platinum-foil, and 

 this was set in a Y-shaped piece of soft sheet copper. The 

 portion of material which received the pressure was about 

 ■J inch long by £ wide ; it consequently had an area of about 

 ^ of a square inch. This limited surface received a pressure 

 in the proportion of over a million pounds to the square inch, 

 or about seventy thousand atmospheres. These, of course, are 

 calculated pressures subject to deduction for friction. The 

 amount lost in this way cannot be determined, but is known 

 to be considerable. 



By the aid of these means the following results were 



