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or larger proportion ; the phlogiston, or inflammable partof the coal, seeming to be the direct tinging substance.When the phlogistic matter was thus diffused throughglass, he did not find it to be affected by continued strongfire, any more than charcoal is when excluded from theair : though some pots of the coloured glass stood for afortnight in the glasshouse furnace, they still retained theircolour ; nor did the most intense fire of a lamp alter it inthe least. How fixt the colour when once united with theglass is, we may judge from the indestructibility, by verystrong fire in open vessels, even of the superficial brownsand blacks which charcoal and foot communicate to glassin its conversion into porcelain : fee page 242.

Mr. Pott, in his neue wichtige pbysicalisch-chymische ma-terien, &c. printed in 1762, observes also that commoncoals give a yellow colour to glass; and that even by ce-mentation, provided the heat is not sufficient to change ittowards porcelain, they stain it of the same colour. Hefays that different coaly matters differ in their tingingpower ; that caput mortuum of soot, and lamp black, an-swer better than common charcoal, and that he has knownsome persons employ the coal of indigo : That the spark-ling coal, which remains in the retort after the rectifica-tion of the thick empyreumatic animal oils, is one of themost active of these kinds of preparations, being as it werethe heavier part of the inflammable substance of the oil,and very rich in colour : That this preparation, powderedand then again burnt a little in a close vessel, is excellentfor tinging glass, and gives yellow, brown, reddish or black-ish colours according to its quantity: That the fritt mustnot be very hard of fusion, for if it is, the strong fire willdestroy the colouring substance before the glass melts ; andthat he has found the following compositions to be nearlythe best, viz. sand 2 parts, alcali 3 parts 3 or sand 2, al-

M m m m 2 cali