270 On the Manufacture of Optical Glass. [1829. 



operate when introduced into soda water or sparkling cham- 

 pagne ; in which cases they cause the gas, which has a tendency 

 to separate from the fluid, to leave it far more quickly and per- 

 fectly than if they had not heen present. 



82. The substance I resorted to for this purpose was platinum 

 in the spongy state. It was chosen as being a body solid at 

 high temperatures, uninfluenced by the glass, easily reduced 

 to powder, and likely to retain its finely divided condition during 

 the operation : its preparation is described in the Appendix. 

 In experiments made expressly to ascertain its action, it was 

 found to assist powerfully in the evolution and separation of 

 the bubbles, and afterwards to sink so completely to the bot- 

 tom, that not a particle remained suspended in the mass. Even 

 stirring does not render it injurious ; for the particles, by that 

 action, are welded to the bottom, and the glass ultimately as free 

 from mixture with them as if they had never been present. 



83. The spongy metal should be perfectly pure. It is easily 

 reduced to powder by rubbing it with a clean finger on clean 

 paper. No attrition with a hard substance should be allowed, 

 as that burnishes the metal, and takes away the roughness, 

 which is highly advantageous in assisting the evolution of the 

 bubbles. When reduced to powder, it should be again heated 

 upon a piece of platinum foil in the flame of a spirit-lamp. 



84-. The quantity of powdered platinum which I have usually 

 employed has been about 7 or 8 grains for every pound weight 

 of glass. But in order to effect its more general and perfect 

 diffusion, I have usually mixed it with ten or twelve times its 

 bulk of pulverized glass. For this purpose, some of the rough 

 glass, the same in composition with that to be perfected, has 

 been crushed small in a clean agate mortar, and the finer parts 

 separated from the coarser on an inclined and shaken sheet of 

 paper. The former have been then mixed little by little with 

 the platinum, and rubbed slightly with the finger, to effect per- 

 fect separation of the metal ; and then the coarser parts have 

 been added, to increase the bulk. In this state it was ready 

 for use. 



85. The time of introducing this prepared platinum is, like 

 the times of stirring, as yet under investigation. It has usually 

 been sprinkled from the platinum ladle (28) over the surface of 

 the well-fused and highly-heated glass, at the period of the first 



