488 M. Carey Lea — Allotrojric Forms of Silver. 



Sometimes the substance will spontaneously pass into a 

 soluble form. A specimen, rendered insoluble by precipitation 

 with ferrous sulphate, after much washing began to run 

 through, not only as a suspension, which often happens, but as 

 a solution, clearing itself, after a day or two, of insoluble por- 

 tions and furnishing a rose-red solution. I have kept this 

 solution in a corked vial for eight months, during which time 

 it has remained unchanged. 



The general properties of this substance can be much better 

 observed in the thin films obtained by brushing the moist 

 substance over paper than in the lumps. The films thus 

 obtained are bright greenish metallic, and this green evidently 

 results from a mixture of blue and yellow, as in some lights 

 the blue, in others the yellow, is most evident. When these 

 films are examined by light reflected from them at a large 

 incidence with the normal and a Nicol's prism or an achroma- 

 tized prism of calc-spar is interposed between the film and the 

 eye, it becomes at once apparent that the blue and yellow light 

 are oppositely polarized. The yellow light is polarized in the 

 plane of incidence, the blue light perpendicularly to that plane. 

 All specimens show the yellow light, but the quantity of blue 

 light is very variable and is directly connected with the amount 

 of washing applied to the precipitate. The more it is washed 

 the more the yellow predominates. To see the blue form in 

 its full beauty, a little of the red solution may be precipitated 

 with a very little magnesium or aluminium sulphate and be 

 thrown on a filter. As soon as the liquid has drained off and 

 without any washing, the deep bronze-colored substance is to 

 be brushed over paper. On drying it has all the appearance 

 of a bright blue metal with a remarkable luster. The mirrors 

 obtained by brushing the substance over glass are so beautiful 

 and so perfect that it seems as if this property might have 

 useful applications, especially for silvering irregular surfaces. 

 Much care, however, would be necessary in the preparation to 

 obtain a permanent product. 



Crystallization. — On one occasion this substance was obtained 

 in a crystalline form. Some crude red solution had been set 

 aside in a corked vial. Some weeks after, it was noticed that 

 the solution had become decolorized, with a crystalline deposit 

 at bottom. The bottle was carefully broken; the deposit, 

 examined by a lens, consisted of short black needles and thin 

 prisms. Evidently the saline matters present had balanced the 

 silver in solution so nearly as not to cause an immediate pre- 

 cipitation, but a very gradual one only. The mother water was 

 drained off and a few drops of pure water were added. No 

 solution took place, the crystals were therefore of the material 

 B, the insoluble form. The contact of pure water instantly 



