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the true principle on which it happens. The followingis the general result os the experiments I have hithertomade upon these subjects.

SECT. I.

Experiments of the fuccejroe changes produced in Green Glass

by baking.

I N order to determine the progress of the visible changeproduced in glass by baking, and the effects of differentdegrees or a different continuance of the heat; a num-ber of pieces of common quart bottles were surroundedwith white sand, in crucibles, which were placed in awind-furnace, built on purpose for experiments of thiskind, consisting of several chambers one over another,with proper apertures in the middle for the ascent of theflame and heated air through each. The crucibles wereleft open, that some of the pieces might be taken outfrom time to time, for discovering how the change wenton : and that the effects of the process might be seen inits full extent, the fire was slowly raised, and continuedfor upwards of forty hours.

Such pieces as were taken out before they became redhot, did not appear to have suffered any change, thoughthey had been kept for several hours in a heat verylittle below ignition. In a low red heat, the changedid indeed take place, but exceeding slowly ■, thosewhich had been exposed for several hours to such aheat being very little altered. In a strong red heat, ap-proaching to whiteness, just not sufficient to make theglass melt, the change went on pretty fast : after anhour’s continuance of this degree of heat, the glass had ac-quired the appearance of porcelain to a considerable thick-ness ; and in two hours longer, the thickest pieces, of the

H h bottoms