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cali 3, calcined borax 1 ; or sand 2, alcali 2, calcined bo-rax 1 : That though saltpetre is hardly, or very spa-ringly, used for yellow glasses, as it too much volatilizesthe colouring substance; yet here for the most part a cer-tain proportion of it, which proportion will be easily foundby trial, is very necessary, for without it the concentratedcolouring matter is apt to make the glass too dark, andeven of an opake pitchy blackness.

That there is any material diversity in the effects of dif-ferent coals, may be justly questioned ; for Mr. More ob-tained, with common charcoal, the same colours, as withthe coals which are here supposed to be of greater excel-lence and activity : it is probable, that the only differenceconsists in their containing different quantities of the in-flammable matter, so that a little more shall be requiredof one kind than of another, for producing the same de-gree of colour in the glass. Nor does the softness or fusi-bility of the fritt appear to be anywise necessary, for myfriend informs me that he has tinged, with coal, glasseswhich were so hard of fusion, that the glassmen could notwork them, whereas the above compositions are all rathertoo soft to be serviceable in the large way.

Page 233. Conversion of green glass into porcelain.

Somk have complained, that on repeating these experi-ments, the change did not succeed; and further enquiryhas Ihewn, that some sorts of green glass are unfit for thisoperation. Green glass has been chiefly made of vegetableashes and sand, brought into fusion together by a strongsire: with this kind, which is the green glass commonabout London, the experiments succeed in the manner de-scribed. In some parts of this kingdom, instead of vege-table assies, the vitrification of the sand has been procured

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