On the Manufacture of Optical Glass, [1829. 



when it forms part of a combination consisting of two propor- 

 tions silica, and one oxide of lead. As yet, the silica I have 

 used has been the flint-glass-maker's sand, obtained from the 

 coast of Norfolk, well washed and calcined. The silicate has 

 been prepared by mixing two by weight of this sand with one 

 of litharge, or with such quantity of nitrate of lead as is equi- 

 valent to one of litharge (16) ; the mixture is put into a large 

 Hessian or Cornish crucible, which being covered over, has 

 been put into a furnace and raised to a bright red heat for 

 eighteen or twenty-four hours. On taking out the crucible, 

 the charge has been found diminished somewhat in bulk, and 

 of a porous structure and appearance like loaf-sugar. It 

 has been freed from the crucible, the outside portions removed, 

 and the pure parts carefully pulverized in a clean Wedgwood 

 mortar. The powder has then been washed over in water, so 

 as to obtain the whole in a fine state of division ; after which 

 it has been dried and preserved in bottles. No sieve should 

 be used in these comminuting operations, nor any reducing 

 or metallic matter brought in contact with the substance. 

 Every care should be taken to avoid contamination. 24* parts 

 by weight of the silicate are equivalent to 16 parts, or one pro- 

 portional of silica, and 8 parts of protoxide of lead. 



21. The advantage of the silica in this combined state de- 

 pends upon the known composition of the substance, its com- 

 paratively easy pulverization, and ready fusion with the other 

 materials. That there is iron in the silica (and the litharge 

 when used) is objectionable ; and the trials for its removal have 

 only been delayed that the investigation of a more important 

 point, namely, a successful process, might proceed. From 

 some brief experiments, I am led to believe that an unexcep- 

 tionable source of silica will be obtained by acting upon this 

 silicate, in a state of fine division, by nitric acid and water, or 

 else by the use of rock crystal. 



22. On some occasions I used pulverized flint glass as the 

 source of silica, conceiving that being already in a fusible state, 

 it must possess an advantage over other silica, in allowing rapid 

 mixture with the other materials. Allowance was made for the 

 oxide of lead present, and the alkali was permitted to pass, as 

 a substance that would probably do no harm. But a striking 

 effect took place, which at once showed the necessity of per- 



