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 manifested. 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 43.05 oxide of lead, 44.3 silica, and 11.75 potassa. But provided 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 will point 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.
110. If a small quantity of flint glass be very finely pulverized
