VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 503 



Exper. A. — As a minute proportion of gold contained in liquors is discoverable 

 by their striking a purple colour with tin, 



1 . Some bright plates of pure tin were put into a solution of platina diluted 

 with water. The plates, in a little time, looked of a dark olive colour, and 

 soon after were covered with a reddish brown matter : the liquor became at first 

 darker coloured, and afterwards by degrees, as the precipitate fell, nearly colour- 

 less ; without exhibiting the least appearance of a purplish hue. 



2. Platina was digested in a quantity of aqua regia insufficient to dissolve the 

 whole ; and the residuum dissolved in a fresh parcel of the menstruum. The 

 two solutions, treated as above, yielded somewhat different phenomena, but no 

 tendency to a purplish cast could be perceived in either. The latter, which 

 looked yellow from not being fully saturated, was, when diluted with water, 

 almost colourless. Yet, on the addition of the tin, it became yellow again, 

 then red, and at length of a dark brownish red considerably deeper than the 

 other more saturated solution. On standing for some time, it grew perfectly 

 clear, depositing a paler, yellowish precipitate. 



3. To determine whether platina was capable of preventing a small proportion 

 of gold from discovering itself on this trial, one drop of a solution of gold was 

 let fall into several ounces of a solution of platina diluted with water. On add- 

 ing some plates of tin, the whole became immediately of a fine purple. 



Remark. It may be proper to observe, that in these kinds of experiments, 

 plates of tin are far more eligible than the solutions of tin usually employed : 

 for the solutions fail of striking a purple colour with solution of pure gold, 

 unless certain circumstances are observed, which are not easily hit upon ; but 

 tin in substance constantly succeeds, and requires no particular precaution. 



Exper. 5. As gold is revived from its solutions by inflammable spirits, the 

 metal gradually arising to the surface, in form of a bright yellow cuticle ; 



1. A solution of platina was mixed with a large proportion of highly-rectified 

 spirit of wine, and exposed for many days to the sun, in a wide-mouthed glass, 

 slightly covered with paper, so as to keep out dust. There was no appearance 

 of any yellow skin ; nor any other alteration, than that the platina had begun 

 to crystallize from the evaporation of the fluid. 



2. A drop or two of a solution of gold being added to a large quantity of a 

 mixture of solution of platina and spirit of wine, and the whole exposed as 

 above to the sun ; a goklen film was in a few days observed on the surface. 



Remark. It follows from this experiment, and the foregoing one with tin,, 

 that platina contains no gold ; and that it cannot, any more than the common 

 metallic or other soluble substances, prevent a small proportion of gold mixed 

 with it from being discoverable. 



