| 154.14 | nitrate of lead contain | 104 | protoxide of lead. | |
| 24.00 | silicate of lead contain | 8 | ditto. | |
| 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 vapour able 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 tire 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
