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



2. Action of heat. 



Allotropic silver is converted by heat to normal silver in 

 either the wet or dry state. 



Dry heat. — When films of allotropic silver on glass are 

 placed in a water desiccator and are kept at 100 C. for eight or 

 nine hours the central portions are converted into the interme- 

 diate form, while at the edges there is a border of grayish 

 white ordinary silver. In fact the change to white silver at 

 the edge commences before the central part is fully converted 

 to the intermediate form. 



At higher temperatures the change is much more rapid and 

 better marked. At 180° C. the first effect is to darken a little : 

 (this is usually the first effect of heat) this continues about five 

 minutes. Continuing the heat for ten minutes more the slight 

 darkening disappears and the film has a bright pure gold color 

 sometimes with a slight salmon tinge. The change to the 

 intermediate form is now complete, the film burnishes yellow 

 and does not react with potassium ferricyanide. It is of in- 

 terest to remark that the color reaction persists as long as there 

 is a trace of unconverted material, so that a film may humish 

 yellow and yet shoio a viell-mar'ked color reaction. This is be- 

 cause most, but not all of the material has undergone conver- 

 sion. 



At 200° C. the film begins in about ten minutes to show a 

 white border and in half an hour or thereabouts it whitens 

 completely. In these experiments the best support for the 

 film is chemically clean glass. Except that for testing the 

 burnishing a film on paper is needed. The paper should be 

 very pure. 



Allotropic silver in the solid form heated to 180° or 190° for 

 about fifteen minutes undergoes a similar change : whereas 

 before it was easily pulverized, it is now almost impossible to 

 reduce it to powder and the powder is yellow instead of being 

 grayish black. 



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

 distilled water maintained at 99° or 100° without actually boil- 

 ing, 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 color 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° C. the film has become pure white. It is not disin- 

 tegrated 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. 



