486 M. Carey Lea — Allotropic Silver. 



When the solution is treated with a very dilute acid, as for 

 example, nitric acid diluted with twenty times its bulk of 

 water, allotropic silver is precipitated in the solid form. It 

 dries with a brilliant metallic surface color of a shade different 

 from the foregoing and somewhat difficult to exactly charac- 

 terize, a sort of bluish steel-gray. 



I do not find that blue allotropic silver (in which is included 

 the green and steel-gray varieties) can be reduced to any one 

 definite type. On the contrary, its variations are endless. 

 Slight differences in the conditions under which the solutions 

 are formed or in the mode of precipitation give quite different 

 products. For example, of ten products obtained with tannin 

 and sodium carbonate in different proportions, several were 

 easily and completely soluble in ammonia, some were slightly 

 soluble and some not at all. Some specimens not at all soluble 

 in water became so by moistening with dilute phosphoric acid : 

 they did not dissolve in the acid but when it was removed they 

 had become soluble in water. On other specimens phosphoric 

 acid had no such effect. Some solutions are scarcely affected 

 by acetic acid, others are partly precipitated, others almost but 

 not quite wholly. The films spread on paper vary very much 

 in their relations to light ; some are readily converted into the 

 yellow intermediate form, whilst others are very insensitive. 

 The least sensitive specimens seemed to be those for which 

 dilute nitric acid had been used as a precipitant. They had a 

 steel-gray color. Precipitation by acetic acid seems to tend to 

 a greenish metallic surface color and greater sensitiveness. 

 Different specimens also vary very much as to permanency ; 

 this character is also affected by the amount of washing re- 

 ceived : thorough washing tends to permanency. 



In some way the blue, gray and green forms seem more 

 closely related to the black or dark gray forms of normal silver, 

 for they tend in time to pass into them, while on the contrary, 

 gold-colored silver, if pure, tends with time to change to 

 bright white normal silver on the surface, with dark or even 

 black silver underneath. 



Action of other Carbonates. 



Tannin is capable of producing allotropic silver, not only in 

 the presence of the carbonates of potassium and sodium, but 

 also with those of lithium and ammonium and also with the 

 carbonates of calcium, magnesium, barium and strontium. The 

 action of the last named carbonate has been more particularly 

 examined. It yields allotropic silver of a dark red color while 

 moist, drying with a rich bluish green metallic surface color in 

 thick films, in very thin films transparent red. It is probable 



