M. C. Lea — Gold- colored Allotropie Silver. 185 



3. Action of Mechanical Force. {Shearing Stress.) 



The slightest application of force suffices to instantly convert 

 gold-colored allotropie silver to normal silver. A glass rod 

 with a rounded end drawn lightly over the surface of a film 

 on paper, leaves a white trace behind it. The force sufficient 

 to cause this change is so slight that one might doubt its reality 

 were it not for the decisive proof immediately at hand. First, 

 there is the characteristic change of color : the film is as yellow 

 and as brilliant as gold leaf ; the line drawn by the glass rod is 

 of pure white silver. Immersing the film in a solution of 

 potassium ferricyanide the white lines simply change to gold 

 color, whilst the film surface on which they are drawn passes 

 through a brilliant succession of colors. (These effects are 

 represented in Plate II, the uppermost figures.) For this use, 

 freshly-made material should be employed, and the film itself 

 should have been freshly spread on pure paper or card and 

 used within a few hours after drying. This because of its 

 easy partial passage, especially in thin films, to the intermedi- 

 ate state, in which it gives a yellow streak. (See also remarks 

 as to partial conversion ante.) 



When the experiment is performed under proper conditions 

 the effect is very striking by reason of the instant conversion 

 of the pure, deep yellow metal to perfect whiteness without a 

 trace of color. 



In an earlier communication to this Journal it was mentioned 

 that, having taken with me on a journey several small vials 

 containing gold-colored silver, they were found at the end of 

 the journey to be all converted into white silver without hav- 

 ing undergone any disaggregation, and retaining the original 

 shape of the fragments. The white silver formed had the fine 

 frosted white color of pure silver. This change was attributed 

 to the friction of the fragments against each other, occasioned 

 by the motion of the journey ; a conclusion that was confirmed 

 by finding that when cotton wool was forced into the empty 

 part of the vial in such a way as to prevent all internal move- 

 ment, the substance could be sent over a four-fold distance 

 without alteration. 



It was also observed (and this is a matter of special interest) 

 that when a partial change had been effected by friction, this 

 alteration went on, although the substance was left perfectly 

 at rest, until it became complete. With time, all solid speci- 

 mens of allotropie silver undergo this spontaneous change to 

 bright white silver, apparently normal silver, even when 

 thoroughly protected from light. 



Out of over twenty specimens in tightly corked tubes, 

 packed in a box and left in a dark closet for a year, not one 



