48 
MR, FARADAY ON THE 
108. A very curious and important influence of alkali in facilitating the tar- 
nish 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 
hut 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 
he disregarded. Here the oxide of lead is 33.28 hundredths of the whole; and 
if it he 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 analysed, and which 
contains 43.05 oxide of lead, 44.3 silica, and 11.75 potassa. But provided the 
alkali he 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 18 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 pro- 
portion 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 tar- 
nished equally with some of the heavier glasses, though containing so much less 
lead, and of such inferior specific gravity. This influence of alkali is asso- 
ciated with, and perhaps directly referrible to, another circumstance affecting 
the liability of change in the glass; 1 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 in an agate 
mortar, then placed upon a piece of turmeric paper, and moistened with a drop 
