Gold-coloured Allotropic Silver. 243 



piece to the other, so that the back of each piece of paper is 

 blackened by silver carried over from the one behind it. 



That the branching grey spots in this way formed are 

 normal silver, is easily proved by immersing the piece in a 

 dilute solution of potassium ferricyanide. The part acted 

 upon by electricity is not affected by the reagent, whilst the 

 rest of the film shows the coloration characteristic of allo- 

 tropic silver. In Plate I. the upper figure shows a slip of 

 paper, at one end of which electricity has been transmitted ; 

 and the figure below, a similar slip that has been subjected 

 to the action of the ferricyanide, showing that where elec- 

 tricity has passed the silver has become normal and is not 

 affected by the reagent. 



2. Action of Heat. 



Allotropic silver is converted by heat to normal silver in 

 either the wet or the 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 

 intermediate form, whilst at the edges there is a border of 

 greyish-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 Rye minutes. Continuing the heat for ten minutes 

 more, the slight darkening disappears and the film has a 

 bright pure gold colour, 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 interest to remark that the colour 

 reaction persists as long as there is a trace of unconverted 

 material, so that a film may burnish yelloiv and yet show a ivell- 

 marked colour reaction. This is because most, but not all of 

 the material has undergone conversion. 



At 200° C. the film begins in about 10 minutes to show a 

 white border, and in half an hour or thereabouts it whitens 

 completely. 



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

 for about 15 minutes undergoes a similar change : where- 

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

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

 being greyish black. 



