1829.] On the Manufacture of Optical Glass. 281 



tating the tarnish of glasses containing oxide of lead, was 

 discovered during the course of these investigations ; and 

 when the quantity of lead in flint glass is increased but a little 

 beyond the ordinary proportions, its effect is powerfully mani- 

 fested. Ordinary flint glass consists of 33*28 oxide of lead, 

 51*93 silica, and 13*77 potassa; the rest of the substances 

 present, being in very small quantity, may be disregarded, 

 Here the oxide of lead is 33*28 hundredths of the whole ; and 

 if it be only a little increased, for the purpose of giving greater 

 dispersive power, the glass is liable to tarnish in an ordinary 

 town atmosphere. Such is the case with a specimen of 

 Guinand's glass, which I have analyzed, and which contains 

 4-3*05 oxide of lead, 44*3 silica, and 11*75 potassa. But pro- 

 vided the alkali be away, the quantity of oxide of lead may be 

 enormously increased ; and a glass containing 64 per cent, of 

 oxide of lead, in combination with 36 per cent, of silica, has 

 not tarnished by an exposure for eighteen months on the same 

 shelves with flint glasses that have tarnished. The following 

 case willpoint out the effect still more strongly : A combination 

 of equal weights of silica and oxide of lead was formed, and 

 the compound has shown no tendency to tarnish in an ordinary 

 atmosphere since February 1828. Eight parts of this was 

 fused with as much pearlash as was equivalent to 1 part of 

 potassa, and a glass was formed which has since become much 

 tarnished. But other 8 parts being fused with 3 parts more 

 of oxide of lead, so as almost to double the proportion of the 

 latter, gave a glass without alkali, which does not yet exhibit 

 the slightest trace of tarnish. 



109. Hence the reason why the absence of alkali has been 

 earnestly insisted upon in the preparation of the ingredients 

 for the heavy optical glasses (18. 24). Hence the reason also 

 why heavy flint glass, as already mentioned, has tarnished 

 equally with some of the heavier glasses, though containing so 

 much less lead, and of such inferior specific gravity. This 

 influence of alkali is associated with, and perhaps directly 

 referable to, another circumstance affecting the liability of 

 change in the glass ; I mean the action of water or of aerial 

 moisture, which is frequently considerable, and appears to be 

 dependent upon the alkali present. 



1 10. If a small quantity of flint glass be very finely pulverized 



