•264 On the Beiiirification of Glass. 



stalli^ations are formed : these are nucU'l co-niposed of small- 

 needles all converging towards the centre. In this state it 

 is no longer glass; it is crystal, which possesses all the 

 ph\'sical properties of mineral substance^ lei'i to themselves. 

 An exact analvsis, made with a certain number ot crys- 

 tals careftdly detached from the mass, would indicate their 

 nature, and throw more light on their formation. 



It often happens that these crystallized nuclei are enve^ 

 loped with a crust which seems foreign to their nature, and. 

 which may be compared to the crust with which flints are 

 covered in the banks of chalk, where they seem to grow. 



Such is the scries of the phenomena exhibited by the 

 devitrification of demi-white glass when it takes place 

 slowly ; but if hastened too much, these phamomcna return 

 to the class of those which mav be observed in gla?* bottles. 

 The common glass in question was tha' in which no earthy 

 substances but wood ashes were employed. There ought, 

 therefore, to be infinije varieties in it, according to the dif- 

 ferent (.■ompo;~':tioiiri. 



It is vciV diflicult to make white glass crystallize or to 

 become devitrified. When well made one may even say 

 that it is not altered by heat a lonfj timo continued ; but 

 that this mav be the case there must be notliing in its com- 

 position except silex, and only the quantity of Hux neces- 

 sary to saturate it. A heat continued as long as possible 

 will not then efiect any other change in the glass than to 

 hardjp. it aix] make it become yellqw. 



When wiiite glass contains a certain quantity of neutral 

 salts or ^lass gall, which the fire has not had time or power 

 to dissipate, there often results during a slow cooling what 

 if* called ^roi.«f, raline, bi'.bbles, and stones, which re 

 suddenl',' and spontaneously formed. 



l"hcsc accidents, their diflerent causes, and the remedies 

 to be applied, are treated of at full length in the first part 

 rtf my work, where I speak of the fusion of glass ; but, 

 t'.iough the explanation of these facts belong entirely to the 

 theory of devitrification, I think it necessary to treat here 

 of the phyenomena arising from the presence of the diflcrent 

 earths. 



Whitp glass contains lime in greater or less quantity, in 

 consequence of jLhe reasons mentioned in speaking ot the 

 diflcrent compositions of this substance, 'i'hislime, when, 

 in excess, crystallizes exceedingly well, ?s reniiyked by 

 I.ovsel. These crystals may be easily distinguished: they 

 are so abimdant that they absolutely obscure the transpa- 

 reucv, They are prisms which seem to flo^t in the midst. 

 ■ ' ' "of 



