MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
37 
city, may be gained by the person who will take a glassful of clear concen- 
trated white sugar syrup, and beat it up with a little air, until a portion of the 
latter is in extremely minute bubbles. If these are allowed to remain undis- 
turbed, it will be observed, that though the larger bubbles rise quickly, and 
the smaller soon after, the smallest will continue for many hours under the 
surface, destroying the pellucidness of the fluid ; and this will be the case 
although there are none of those descending currents, resulting from difference 
of temperature, which in the glass assist in retaining the bubbles beneath the 
surface. 
81. From the great length of time which it required to liberate the bubbles 
even from small pieces of glass, and when no stirring was practised, I was 
induced to conclude that the evolution of gaseous or vaporous matter had not 
ceased upon the first fusion of the materials, but that the glass itself when 
highly heated continued to evolve small portions for some time. It occurred to 
me also, that in that case its formation might be hastened and the final sepa- 
ration advanced by mixing some extraneous and insoluble substance with 
the glass, to act as a nucleus, just as pieces of wood, or paper, or grains of 
sand, operate when introduced into soda water or sparkling champaign ; 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 platina 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 equally free from 
mixture with them. 
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 evolu- 
