Allotropic Silver. 501 



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 speci- 

 mens seemed to be those for which dilute nitric acid had 

 been used as a precipitant : they had a steel-grey colour. 

 Precipitation by acetic acid seems to tend to a greenish 

 metallic surface, colour, and greater sensitiveness. Different 

 specimens also vary very much as to permanency; this cha- 

 racter is also affected by the amount of washing received : 

 thorough washing tends to permanency. 



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

 closely related to the black or dark grey forms of normal 

 silver ; for they tend, in time, to pass into them ; whilst, on 

 the contrary, gold-coloured silver, if pure, tends, with time, 

 to change to bright white normal silver on the surface with 

 dark, or even black, silver underneath. 



Actum of other Carbonate*. 



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 par- 

 ticularly examined. It yields allotropic silver of a dark-red 

 colour whilst moist, drying with a rich bluish-green metallic 

 surface-colour in thick films, in very thin films transparent 

 red. It is probable that the substances with which tannin 

 produces these reactions would be further increased bv 

 investigation. 



I have found some additional modes of production of these 

 forms of silver, modes which are very curious and inter- 

 esting. They are now being studied, and will be reported on 

 hereafter. 



Nature of the "Intermediate Substance." 



It has been mentioned in previous papers that when allo- 

 tropic silver is converted into normal silver by the action of 

 heat, it passes through a perfectly well-marked intermediate 

 state. In this state it retains the gold-yellow colour and high 

 lustre, but none of the other properties of the original form. 

 Oxidizing and chlorizing agents show nearly the same indif- 

 ference as with ordinary silver. Whilst allotropic silver is 

 soft and easily reduced to powder, the intermediate sub- 

 stance is hard and tough. When a glass rod is drawn over a 

 film of allotropic silver, it leaves behind it a white trace of 

 ordinarv silver. The intermediate substance shows no such 



