MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
15 
Hence the materials for any quantity of glass can be easily calculated ; and if 
the above parts be ounces, about 9lbs of glass will result. The nitrate of lead 
is then to be broken small in a clean mortar, and the other ingredients well 
mixed with it in basins, the use of metal or dirty implements being carefully 
avoided. 
26. The mixture is next melted, and made into rough glass. This prepa- 
ratory operation is necessary, because from the quantity of vapourable matter 
which is disengaged in this part of the process, the materials, if put at once 
into the finishing vessel and furnace, might boil over and do injury ; and the 
acid nature of the vapours themselves, if it did not occasion harm by acting on 
neighbouring iron and other parts of the furnace, would at least cause inconve- 
nience. It is effected in a furnace, which will be particularly described in 
the Appendix to this paper. It will be sufficient here to state, that being a 
close furnace, the part immediately beyond the fire-place forms a horizontal 
chamber, covered above by an iron plate having large circular holes ; these 
allow crucibles to pass through them, and to stand supported on the bottom of 
the chamber, whilst their edges rise above the upper iron plate. In this way 
the fire is applied very generally to the crucibles, whilst their mouths are altoge- 
ther exterior to the furnace, so that the introduction of any reducing or colouring 
impurity from the fire is prevented, and the greatest facility in introducing the 
mixture, of watching its fusion, of stirring the glass, and finally of ladling it out, 
is obtained. The holes through which these crucibles are inserted are five or 
six in number ; they are never all in use at once, and those out of use are 
covered by crucible covers. The heat is not given altogether by flame ; but 
whilst coal is used in the fire-place, coke is applied between the crucibles, being 
introduced for that purpose, and arranged through the unoccupied holes. The 
iron top of the furnace is covered by a second iron plate, or, what is better, by 
earthenware plates, to retain the 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 any thing 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 crucible into 
