266 On the Manufacture of Optical Glass. 



equal time. The specific gravities of each after the experi- 

 ments were as follows : 



Highest heat . 5-4206 5-4211 5-4203 Mean sp. gr. 5*42066 

 Intermediate heat 5-4253 5-4242 5-4255 5-42500 

 Least heat . . 5-4258 5'4262 5'4235 5-42516 

 Original glass . 5 -4247 5-4261 . .7.: 5-42540 



72. Here, notwithstanding the irregularities between the 

 similar experiments, there seems, from the comparison of the 

 mean specific gravities, to be a gradual though minute diminu- 

 tion of density, as the glasses have been more powerfully heated ; 

 and I found also, that when glass was so well stirred as to leave 

 no doubt that it was thoroughly well mixed, yet being left in 

 the furnace at a high temperature for eight or nine hours, it 

 contained striae. 



73. On the other hand, first to render the glass perfectly 

 free from bubbles and clear, and then to stir out the irregula- 

 rities of composition, I have not found to be a practicable pro- 

 cess ; because the stirring, in the manner in which I have yet 

 performed it, tends to introduce bubbles into the glass ; and 

 though these are small, still they are objectionable. Hence 

 a mixed process has been adopted, which, as I have before 

 stated, is subject to correction from future experiments. To 

 render the process as far as it has been carried sufficiently in- 

 telligible to others, I will first describe the circumstances con- 

 nected with stirring, and their influence upon striae ; and after- 

 wards, the plans adopted for the dispersion of bubbles. 



74. It is not a small degree of stirring and agitation which 

 is sufficient to make a fluid of mixed materials homogeneous ; 

 especially when the mixture is not exceedingly fluid, but 

 has, like tar or syrup, a considerable degree of tenacity. 

 An idea of the extent to which it must be carried, and of the 

 general nature of striae in fluids, may be gained by taking a 

 glass full of clear saturated syrup, made from white sugar, 

 putting a few drops of water into it, and stirring the whole to- 

 gether. It may then be remarked how slow the striae are in 

 disappearing ; and when they are apparently destroyed, if the 

 whole be left for some hours, it will frequently happen that a 

 separation will take place into a lower heavy, and a superincum- 

 bent light portion, which, when stirred together again, produce 

 striae. In the glass, the stirring must be in the utmost degree 



