240 Mr. M. Carey Lea on 



of a reducing agent, as for example with potassium ferri- 

 cyanide and permanganate and with ferric chloride &c«, its 

 behaviour differs from that of ordinary silver chiefly in 

 showing greater activity. The difference is rather of degree 

 than of kind. The formation by these reagents of coloured 

 films will be described at the end of this paper. 



Intermediate Fokm. 



Allotropic silver presents itself in an almost endless variety 

 of forms and colours — gold-coloured, copper-coloured, blue, 

 and bluish green (these last in thin films), red or purple. 

 Most of these varieties seem to be capable of existing in two 

 conditions, of which one is more active than the other. 



If we coat a chemically clean glass plate with a film of gold- 

 coloured allotropic silver, let it dry (first in the air, then for 

 an hour or two in a stove at 100° C), and then heat the 

 middle of the plate carefully over a spirit-lamp, we shall 

 obtain with sufficient heat a circle of whitish grey with a 

 bright, lustrous, golden-yellow ring round it, somewhat 

 lighter and brighter than the portion of the plate that has 

 not been changed by heat. This ring consists of what I 

 propose to call the " intermediate form." 



Its properties are better seen by using a film formed on 

 pure paper, one end of which is heated over a spirit-lamp 

 to a temperature just below that at which paper scorches. 

 The change is sudden, and passes over the heated portion 

 of the surface like a flash. 



Examining the changed part, we find: — 



1st. That it has changed from a deep gold to a bright 

 yellow-gold colour. 



2nd. When subjected to a shearing stress it does not 

 ivhiten or change colour in the slightest degree. 



3rd. It is much harder, as is readily perceived in bur- 

 nishing it. 



4th. It no longer shows the colour reaction with potassium 

 ferricyanide and ferric chloride, changing only by a slight 

 deepening of colour. 



Of these characteristic changes the second is the most 

 remarkable. The gold-coloured silver in its original con- 

 dition changes with singular facility to white silver : almost 

 any touch, any friction, effects the conversion. If the paper 

 on which a film is spread is creased, the crease is found to be 

 grey. Exposure to heat or to light destroys this capacity for 

 change, and it is often lost by mere standing (even though 

 protected from light) for a few weeks. This evidently indi- 



