1S29.] On the Manufacture of Optical Glass. 247 



heat. The crucibles are of pure porcelain ware, and as thin 

 as they can be obtained. The covers for them are evaporating 

 dishes, considerably larger than the mouths of the crucibles : 

 being turned upside down, they rest, when in their places, upon 

 the neighbouring earthenware plate ; not touching the crucibles, 

 but preventing anything from falling into them, and preventing 

 the vapours from passing into the room. The latter are, by the 

 draught of the chimney, drawn through by the sides of the cru- 

 cible into the furnace, and carried away up the flue, so as to 

 occasion no annoyance to the operator. The covers are slung 

 by a piece of platinum wire, which, being passed across the mid- 

 dle on the outside, is bent at each end round the edges, so that 

 a rod of iron slightly curved at the extremity easily suffices to 

 remove them when the crucible is to be opened. Great care is 

 always taken to put them in clean situations, and that in their 

 removal nothing shall fall from them into the glass. 



27. This furnace is found to be very effectual in its action ; 

 being connected with a high flue governed by a damper, great 

 command of the temperature is obtained. The crucibles be- 

 fore being used are examined as to soundness ; their tem- 

 perature is raised gradually, and should not be above a dull 

 red heat when the operation commences. The mixture already 

 described (25) is then introduced, and the crucible covered ; 

 decomposition of the nitrate of lead instantly commences ; the 

 boracic acid loses its water, all the fixed elements unite ; and it 

 is remarkable that though a considerable quantity of boracic 

 acid usually sublimes with the water when the latter is driven 

 off from its crystals unmixed with other substances, yet scarcely 

 a trace seems to evaporate in the present instance, in conse- 

 quence of the presence of the oxide of lead. 



28. The heat should not be raised too high or the operation 

 hastened, and then the ebullition will proceed very gradually 

 and favourably, the rough materials being by degrees converted 

 into glass. Before the first charge is entirely melted a second 

 is put in, and when that is fused down, sometimes a third, ac- 

 cording to the quantity of glass present and the soundness of 

 the crucible. When all is fused, the temperature is allowed to 

 rise, but not too much, lest action upon the crucible to a serious 

 extent should occur ; the glass is then well agitated and mixed 

 by a platinum rake or stirrer, to be described hereafter. Finally, 



