1829.] On the Manufacture of Optical Glass. 239 



ent. Oxide of lead enters into its composition to the amount 

 of one third of its weight, or more, and by its presence gives 

 that proportion of refractive and dispersive power, which makes 

 the glass valuable in conjunction with crown or plate : this it 

 does in consequence of its own powerful action on light ; and 

 it makes the glass heavy also, because of its own great specific 

 gravity. A third property belonging to it, namely, its high 

 fluxing or dissolvent powers, it also confers upon the glass. 

 Now these three properties are unfortunately very conducive 

 to the formation of striae. If the least difference in composi- 

 tion exists between one part and another it becomes evident, 

 because of the great difference between the qualities of the 

 oxide of lead and the other ingredients ; and a variation in 

 proportions which in crown or plate glass would produce no 

 sensible effect on the naked eye, would, in flint glass, form 

 strong strise. Hence it is required that the mixture be in this 

 case far more perfect than in the other glasses ; and yet it 

 unfortunately happens that every thing tends to make it much 

 less so. The oxide of lead is so heavy a material, and at the 

 same time so fusible, that it melts and sinks to the bottom, 

 leaving the lighter materials to accumulate at the top : and so 

 imperfect are the means of mixture, under ordinary circum- 

 stances, that glass of very different specific gravity is procured 

 from the bottom and top of the same crucible. The following 

 are some cases of this kind, from pots containing glass not more 

 than six inches in depth, made from the usual materials, and 

 retained at a full heat for twenty-four hours : 



Top 3-38 3*30 3-28 3'2l 3-15 3-73 3'85 3'81 3-31 3-30 



Bottom. ..4-04 3'77 3-85 3'52 3'80 4*63 4-74 4-75 3'99 3'74 



These differences are great, and selected for illustration; 

 but from appearances there is little reason to doubt that the 

 same state of things, though not to such an extent, occurs in 

 every pot of flint glass made in the ordinary way. 



8. Another curious illustration of the predominance of oxide 

 of lead at the bottom is shown in many of our specimens, which 

 have been broken through vertically : they have been affected 

 by sulphuretted vapours and tarnished ; but the tarnish has 

 occurred only at the bottom, where the lead is abundant, and 

 is there very strong, whilst there is no appearance of it towards 

 the top. 



