418 M. C. Lea — Endothermic Decompositions by Pressure. 



Silver Tartrate. — When spread in a very thin skin over the 

 mortar each sharp stroke of the pestle left a black line behind 

 it. This is a strong constrast with the complete resistance of 

 this substance to simple pressure. 



Silver Carbonate. — Action similar. 



Silver Citrate. — Blackens very easily. 



Silver Oxalate. — At least as easily. 



Silver Arsenate. — Nearly as easily. 



Silver Sulphite. — Effect visible in live minutes and gradually 

 increasing. Very well marked. 



Silver Salicylate. — No silver salt appears to be so easily re- 

 ducible as this. Every sharp stroke of the pestle leaves a 

 brown mark behind it. 



Silver Orthop/iosphate. — Affected easily. After the phos- 

 phate has been a good deal reduced the unchanged part may 

 be dissolved out with ammonia. The black residue after 

 washing readily dissolves in dilute nitric acid and the solution 

 gives a white cloud with hydrochloric acid. 



Potassium Ferridcyanide. — A crystal of the pure salt 

 sharply ground in a mortar becomes in portions brown and in 

 others blue. The quantity used must, as indeed in all of the 

 above cases, be small, one or two decigrams. If a little dis- 

 tilled water be added an insoluble blue powder is left behind 

 and the solution formed strikes a blue color when added to one 

 of ferric alum. This indicates that the decomposition is two- 

 fold. The experiment is quite a striking one and the result is 

 easily obtained. 



(ii) 



This form of mechanical force, shearing stress, may be ap- 

 plied to effect endothermic change in other ways. A very 

 simple, and at the same time very efficient, method is that of 

 pressure with a glass rod. Pure strong paper is to be imbued 

 with a solution of the substance, if it is soluble, or if not, it is 

 to be made into a paste with water and then applied with a 

 brush. The paper is to be then very thoroughly dried and is 

 to be laid upon a piece of plate glass. Characters are to be 

 marked on it with the end of a glass rod that has been rounded 

 by heat, using as much pressure as is possible without tearing 

 the paper. 



More than twenty years ago I was able to show that marks 

 made in this way on sensitive photographic films could be de- 

 veloped, as an invisible image had been impressed. That, 

 however, is a somewhat different matter from actual and visi- 

 ble decomposition following each stroke of the rod, a result 

 which may be obtained with various salts of gold, mercury, 

 silver and other metals. 



