M. C.Lea — Gold-colored Allotropic Silver. 181 



In those reactions in which allotropic silver acts the part of 

 a reducing agent, as for example with potassium ferricyanide 

 and permanganate and with ferric chloride, etc., its behavior 

 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 colored films will be de- 

 scribed at the end of this paper. 



Intermediate Form. 



Allotropic silver presents itself in an almost endless variety 

 of forms and colors, gold-colored, copper-colored, 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- 

 colored 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 ]amp, we shall obtain with 

 sufficient heat a circle of whitish gray 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 " inter- 

 mediate 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 tem- 

 perature 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 color. 



2d. When subjected to a shearing stress it does not whiten 

 or change color in the slightest degree. 



3d. It is much harder, as is readily perceived in burnish- 

 ing it. 



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

 ferricyanide and ferric chloride, changing only by a slight 

 deepening of color. 



Of these characteristic changes the second is the most 

 remarkable. The gold- colored silver in its original condition 

 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 

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

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

 tected from light) for a few weeks. This evidently indicates 

 some remarkable molecular change. It will be noticed that 

 the anomaly lies in this, that pressure instantly effects the 



