Chap. 14] Art of Glifs-maling. 95 



earthy fubftances, of which the flinty earth is the oeft, 

 and for that reafon is called verifiable. But as earths 

 cannot eafily be fufed without mixture, it is neceffary 

 to add certain fubftances which may promote the 

 vitrification. In the making of ordinary glafs, two 

 parts of fand, or other filiceous matter, are mixed with 

 about one of fixed alkali. If the glafs is not required 

 to be tranfparent, impure alkali, not freed from the 

 alhes, is employed; but in making the finer and moll 

 tranfparent kinds of glafs, care is taken to purify both 

 the earth and the alkali. One of the chief points to 

 be obferved in the making of giafs is to heat the mix- 

 ture gradually, fo that the elaftic fluids may efcape 

 before the materials cohere, as the ingredients arc 

 otherwife apt to be fo fwelled by the difengagement 

 of air as to be loft by flowing over the fides of the 

 veflcl in which they are heated. In order to prevent 

 this, due proportions of fand and alkali are mixed to- 

 gether, and expofed, during a confiderable time, to 

 a red heat, not intenfe enough to melt them. By 

 this calcination the inflammable matters, which would 

 have impaired the colour of the glafs, are confumed, 

 and the air expelled. This firft mixture of the mate- 

 rials of glafs, after being treated in this manner, is 

 called the fritf. 



The due degree of heat is an efTential point in 

 making of glafs : it ought not only to be very ftrong, 

 but alfo maintained during a long time. In great 

 manufactories the glafs is kept fufed during ten or 

 twelve hours before it is taken out of the pots. The 

 ingredients are thus more completely melted, and more 

 thoroughly mixed, and the "appearance of threads or 

 veins is in fome meafure prevented, which proceed 

 from the different denfity of different portions of the 

 glafs, and which therefore ad differently on the rays 



of 



