246 On the Manufacture of Optical Glass. [1829. 



154*14 nitrate of lead contain . . . 104 protoxide of lead. 



24*00 silicate of lead contain . . . { 



L 16 silica. 



42'00 crystallized boracic acid contain 24 dry boracic acid. 



152 glass. 



Hence the materials for any quantity of glass can be easily 

 calculated; and if the above parts be ounces, about 91bs of 

 glass will result. The nitrate of lead is to be broken small 

 in a clean mortar, and then 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 preparatory 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 inconvenience. 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 

 altogether 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 intro- 

 duced 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 



