Brimstone is a vein of the earth and hath much air and fire in its composition. Of brimstone there are four kinds. One is called _vivum_, the which when it is digged, shineth and flourisheth, the which only among all the kinds thereof physicians use. Avicenna means that brimstone is hot and dry in the fourth degree, and is turned into kind of brimstone in part of water, of earth, and of fire, and that brimstone is sometimes great and boisterous and full of drausts, and sometimes pure white, clear and subtle, and sometimes mean between both. And by this diverse disposition, divers metals are gendered of brimstone and of quicksilver. Glass, as Avicen saith, is among stones as a fool among men, for it taketh all manner of colour and painting. Glass was first found beside Ptolomeida in the cliff beside the river that is called Vellus, that springeth out of the foot of Mount Carmel, at which shipmen arrived. For upon the gravel of that river shipmen made fire of clods medlied with bright gravel, and thereof ran streams of new liquor, that was the beginning of glass. It is so pliant that it taketh anon divers and contrary shapes by blast of the glazier, and is sometimes beaten, and sometimes graven as silver. And no matter is more apt to make mirrors than is glass, or to receive painting; and if it be broken it may not be amended without melting again. But long time past, there was one that made glass pliant, which might be amended and wrought with an hammer, and brought a vial made of such glass tofore Tiberius the Emperor, and threw it down on the ground, and it was not broken but bent and folded. And he made it right and amended it with an hammer. Then the emperor commanded to smite off his head anon, lest that his craft were known. For then gold should be no better than fen, and all other metal should be of little worth, for certain if glass vessels were not brittle, they should be accounted of more value than vessels of gold.