ART OF? CLASSMAKING. 55 



thickness. I brought away a piece of it, which T showed at 

 the public lectures of the Academy of Dijon, in the col- 

 lection belongint^ to which it was deposited. 



It was already well known, that olass, in the composition The lead ren- 

 of which a large proportion of oxide of lead is employed, ^^1^^^^^:^^ 

 does not easily afford a homoi^encous mass, because the form. 

 denser parts are not retained by an affinity capable of pro- 

 dacing an equilibrium ; and hence the difficulty of ob- 

 taining flint-glass free from streaks. But such a speedy 

 •nd complete precipitation is a solitary instance, arising 

 from a combination of circumstances, which we can scarcely 

 hope to reproduce. 



From what has been said, we can scarcely doubt, that the Its streaks a 

 streaks, from which glass abounding in oxide of lead is ^^^^^ dl 

 seldom free, arise from a commencement of eliquation. quaiion. 

 The horizontal position of these streaks proves it ; for they 

 are not distinctly perceptible, except the light comes to the 

 eye in a direction parallel to the zones of unequal density. 

 I have a piece of flint-glass, manufactured also under my 

 own eye, which is three cent. [1*18 in.] thick, which any 

 one, not* apprised of the contrary, would suppose to be 

 perfectly good, because the division is softened down. 



Obs. II. Trials of a crucible-mould for annealing large 

 masses of glass. 



In the various experiments made at the plate-glass manu- Stone mould 

 factory at Rouelle for the same purpose, a hard calcareous [^J"g^"^^^jg"8^f 

 stone, cut iu the shape of a circular crucible, was at first gla«. 

 employed. It was supposed, that, when it had been con- 

 verted into lime by a graduated fire.without having its shape 

 altered, it would hold the refined glass; so that, bounding 

 the bottom accurately, the glass would be annealed by 

 cooling slowly in it, as in an annealing furnace. The result Effects, 

 was a mass full of large blebs throughout its substance, 

 and on its surface. 



"With the same view trial was made of a crucible-mould Pottery 

 made of the best potter's rlay, and baked as hard as pos- niould. 

 sible. The glass was perfectly refined in this, and retained Jesuit, 

 ita homogeneousuess in annealing; but the mass, 7 cent. 



['2-75 



