506 PHILOSOPHICAL TRANSACTIONS. [aNNO 1754. 



plates of pure tin, was triturated an an iron mortar with 8 times its quantity of 

 common white glass, the mixture put into a crucible, which was closely luted, 

 and placeid in a wind furnace. The fire was gradually raised, and kept up ex- 

 tremely strong for about 10 hours; when, the crucible being taken out and 

 broken, the matter appeared of a dark blackish colour, untransparent, easily 

 friable; interspersed with a bright whitish matter, apparently metallic. 



Remark. It is probable, that this metallic matter was the platina; and that 

 the glass owed its opacity and dark colour, not to this mineral, but to the tin 

 in the precipitate, some particles of iron abraded from the mortar, or other ac- 

 cidental causes. 



2. A quarter of an ounce of a precipitate of platina, made by alkaline salt, 

 was ground in a glass mortar with 12 times its weight of white glass; and com- 

 mitted to the same fire as the foregoing. The result was a compact, cloudy 

 glass, pretty transparent in thin pieces, covered in part with a thin whitish coat. 

 Towards the upper part, and all round the sides, were observed several particles 

 of metal; which appeared to the eye like bright platina, and proved hard to the 

 point of a knife. 



Remark. Nor does the glass here seem to have received any thing from the 

 platina; the change being no other than what white glass is found to undergo 

 from a slight impregnation with inflammable matter. 



General Remarks. It appears from the experiments related in this paper, that 

 platina, like gold, is not acted on by the simple acids,* which dissolve every 

 known metallic body besides; that aquae regiae, the solvents of gold, prove like- 

 wise menstrua for platina; and that consequently the common methods of assay- 

 ing or purifying gold by aquafortis, aqua regis, or the regal cement, can no 

 longer be depended on; that it differs from gold, in giving no stain to the solid 

 parts of animals, not striking a purple colour with tin, not being revived from 

 its solutions by inflammable spirits, not being totally precipitable by alkaline salts; 

 that in certain circumstances it throws out gold from its solutions; that these 

 properties afford means of distinguishing a small proportion of gold mixed with 

 a large one of platina, or a small proportion of platina with a large one of gold; 

 and that platina contains no gold, excepting the few particles distinguished by 

 the eye; that platina is precipitated from its solutions by the vitriolic acid, and 

 by the metallic substances, which precipitated gold, though scarcely totally by 

 any; and that its precipitates resist vitrification, and this perhaps in a more per- 

 fect manner than precipitates of gold itself. 



PAPER III. 

 The two former papers have given an account of the habitus or relation 



• As mentioned in a former note, it is soluble in the oxyrauriatic acid. 



