244 Mr. M. Carey Lea on 



Moist Heat. — A film spread on pure paper and placed in 

 distilled water maintained at 99° or 100° without actually 

 boiling, at the end of half an hour is converted almost wholly 

 to the intermediate form. It burnishes pure yellow, but still 

 shows traces of the colour reaction. 



A better plan of operating is to immerse a film spread on 

 glass in distilled water, and to place it in a desiccator with a 

 water-jacket. After keeping for twenty-four hours close to 

 100° 0., the film has become pure white. It is not disinte- 

 grated by the change, but may be detached from the glass in 

 films exactly resembling ordinary silver leaf. 



The effects of heat are shown in Plate I., lower pair of 

 figures. 



3. Action of Mechanical Force (Shearing Stress). 



The slightest application of force suffices to instantly con- 

 vert gold-coloured allotropic 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 colour : 

 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 colour, whilst the film surface on 

 which they are drawn passes through a brilliant succession of 

 colours. (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 intermediate state, in which it 

 gives a yellow streak. (See also remarks as to partial con- 

 version, 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 colour. 



In an earlier communication to the ' American Journal of 

 Science,' it was mentioned that, having taken with me on a 

 journey several small vials containing gold-coloured silver, 

 they were found at the end of the journey to be all converted 

 into white silver without having undergone any disaggrega- 

 tion, and retaining the original shape of the fragments. The 



