9 6 ./#/ <?/ Glafs -mating. [Book VI. 



of light. The imperfections of glafles are moft fcn- 

 fibly felt in the conftruction of optical inftruments. 



Good glafs, although kept in fufion for a long time 

 in a great heat, is never perfectly liquid. It is always 

 ibmewhat thickj and when taken out may be drawn 

 . into fine threads. The great ductility and flexibility 

 of red hot glafs fits it for being reduced into any form. 

 In general the glafs is taken out of the pots in which 

 it is fufed by means of an iron tube. When a fufficienr, 

 quantity of the glafs is collected at the extremity of 

 the tube, the workman begins to blow through it, an'cl 

 thus inflates the glafs. He occafionally rolls it, in 

 order to form it into a cylinder, a cone, &c. and if 

 it becomes too cold, he heats it by holding it before 

 the mouth of the furnace. Glafs, in its tough ftate, 

 may be cut with (hears, bended with pincers, prefled 

 into moulds, and wrought, in a variety of methods 

 dependent on thefe properties, into the vaft variety of 

 forms which it is made to afiume. Glafs veffcls as 

 foon as made> are carried to an oven, in which they* 

 are gradually cooled j for without this procefs, which 

 is called atmealling, they would either break in the cool- 

 ing, or be liable afterwards to be broken by the flighteft 

 force. 



The principal defects of glafs are colours, veins* 

 and bubbles. The colours which generally injure the 

 co'mmon alkaline glafs are mades of green, blue, and 

 olive. Thefe are moft effectually removed by the 

 addition of a fmall quantity of manganefe. The caufe 

 of the veins has been already explained, and that of the 

 bubbles depends on the impe-rfeft expu-lfion of air. 



The addition of calx of lead to glafs renders it mucn 

 more denfe, and lefs liable to be broken. Artificial 

 gems are only the beft kinds of glafs, coloured with 

 different metals 5 but the modes of applying the feveral 



metals 



