[ 6 2I ]

that a small quantity of it, mixed with vitreous com-positions, gives them a notable green tinge. He observesthat the imperfection of flints, &c. for the use of touch-stones, consists in their hardness, which occasions them togive too great brightness to the metalline stroke, so that itscolour, and consequently the proportion of alloy, cannotbe exactly judged of.

Page 172, &c. Cajjius’s precipitate—Ruby glass.

Cassius does not appear, from his treatise de auro, tohave been the discoverer either of the precipitation by tin,or of the tinging of glass by the precipitate. He describesthe preparation of the precipitate, and slightly mentions itsuse in this intention; but gives no account of the mannerof employing it, nor any practical hint in regard to theoperation, except that in speaking of the smallness of theparts of gold, he fays that one dram of gold duly preparedwill tinge ten pounds of glass.

A process for making the ruby glass has been communi-cated to me by an artist, who fays he was allured it camefrom Kunkel, and that he had found it a good one forenamelled colour, but had never tried it for glass. Thegold is directed to be dissolved in a mixture of one part ofspirit of salt and three of aquafortis; and the tin, in amixture of one part of the former of these acids with twoof the latter. The solution of gold being properly dilutedwith water, the due proportion of which is to be found bytrials made on a small quantity as mentioned in page 176,the solution of tin is added, and the mixture suffered tostand till the purple matter has settled to the bottom. Thecolourless liquor is then poured off; and the purple sedi-ment, while moist and not very thick, is thoroughly mixedwith powdered flint or sand: this mixture is well ground

L 1 11 2 with