§6 Mr. M. Carey Lea on Endotfiermic 



Platinic Chloride. — Gradually darkened in a very marked 

 way, finally becoming blackish. 



Ammonium Platinichloride gave same result. 



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 contrast with the complete resist- 

 ance of this substance to simple pressure. 



Silver Carbonate. — Action similar. 



Silver Citrate. — Blackens very easily. 



Stiver Oxalate. — At least as easily. 



Silver Arsenate. — Nearly as easily. 



Silver Sulphite. — Effect visible in five minutes and gradu- 

 ally increasing. Very well marked. 



Silver Salicylate. — No other silver salt appears to be so 

 easily reducible as this. Every sharp stroke of the pestle 

 leaves a brown mark behind it. 



Silver Or thophosphate.— 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 

 distilled water be added an insoluble blue powder is left be- 

 hind, and the solution formed strikes a blue colour when 

 added to one of ferric alum. This indicates that the decom- 

 position is twofold. The experiment is quite a striking one, 

 and the result is easily obtained. 



II. 



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

 applied to effect endo thermic 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. This 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 with- 

 out tearing the paper. 



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

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

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



