[ iBo ]

green, brown and pale yellow in different parts, greatly re-sembling the coat of some pebbles : in this state, lookedthrough against the fun, it appeared of a beautiful rubycolour, and on breaking it, the internal part was foundthroughout of an uniform dark red when looked downupon, and of the ruby red when placed between the eyeand the light. A large piece being continued under themuffle for four hours, its figure was found scarcely al-tered, the coat was much thicker and beautifully veinedwith various colours, which were all lost in a gloriousred when the piece was viewed between the light.

All I have been able to learn in regard to the prepara-tion of this glass is, that the quantity made at once isabout fix hundred weight; that the tinging matter ismixed with the vitreous materials before they are put intothe melting pot, the mixture being brought to the glass-house in tubs ; that the matter is not stirred in fusion ;and that it is kept no longer in the fire than is necessaryfor perfecting the glass, which, as soon as fine, is castinto a kind of bricks. Some imagine that this glass hasno mixture of calx of lead, of which a large proportionis used in the composition of the common flint glass,and that the principal vitrefying ingredient is nitre : othersjudge it to be composed of the same materials as thecommon fort; its weight seeming to be a proof of itscontaining lead, for it is found to be nearly of the famespecific gravity with flint glass, which is greater than thatof the glasses made without lead in the proportion ofabove six to five. This point I have determined in amore satisfactory manner : four hundred grains of theglass, made red hot and quenched in water, were reducedinto powder, and mingled with about twice as muchblack flux and a little alkaline salt: the mixture beingmelted in a crucible, and the vessel suffered to cool, a

i lump