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270 operate when introduced into soda water or sparkling champagne; in which cases they cause the gas, which has a tendency to separate from the fluid, to leave it far more quickly and perfectly than if they had not been 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 bottom, 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 linger, to effect perfect 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