1829.] On the Manufacture of Optical Glass. 269 



and state of that in the tray; but during its examination, the 

 experimenter must carefully refrain from touching it ; for if 

 the finger, or any other organic substance, come into contact 

 with it, the next time the instrument is immersed in the ignited 

 glass, the part touched will produce bubbles. It is therefore 

 of importance that the stirrer be preserved perfectly clean from 

 one stirring to another, for which purpose it may be deposited 

 so that the platinum shall be received in an evaporating basin, 

 the mouth of which is afterwards covered over. 



80. In entering upon the considerations relative to the bub- 

 bles, it will be evident, from the nature of the materials and the 

 quantity of elastic matter originally present, that these air cavi- 

 ties are at first very numerous. The larger ones soon ascend to 

 the surface, and, breaking, are dissipated without inconvenience ; 

 but the smaller ones rise with far less readiness, and the smallest 

 have so little power of elevation, that the general currents in 

 the liquid appear sufficient to carry them downwards, or in any 

 other direction, and thus retain them for any period within the 

 mass. A useful idea of the length of time required for very 

 minute bubbles to ascend through a fluid having some tenacity, 

 may be gained by the person who will take a glassful of clear 

 concentrated 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 undisturbed, 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 cur- 

 rents, 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 libe- 

 rate the bubbles even from small pieces of glass, and when no 

 stirring was practised, I was induced to conclude that the evo- 

 lution 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 separation 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 



