164 C. J. WHITE. 
due to the presence of ferrous silicate, i.e., to whiten the 
glass; the name pyrolusite is given to the black oxide of 
Manganese on account of this property, and the French call 
it ‘‘Savon des verriers’’ for the same reason; it has been 
employed for this purpose from the earliest times, and 
its use is mentioned by Pliny. The manganese peroxide acts 
partly by oxidising ferrous silicate to ferric silicate, which 
possesses much less tinctorial power than the former, and 
partly by the amethyst colour of the manganese silicate 
(which is complementary in tint) neutralizing the green. 
The presence of manganese in glass can often be seen by 
the amethyst tint which window glass acquires when 
exposed to the sun’s rays, and it is also seen in fragments 
of tumblers and other ware made of flint glass, which have 
been thrown out and left exposed to the sun. This change 
in colour is due to the green ferrous silicate having been 
converted into the reddish ferric silicate; the amethyst 
colour of the manganese silicate is then no longer neutra- 
lized, and accordingly the glass shows a more or less deep 
purple tint. 
Average Sample.—The first analysis was made upon some 
fragments of the mixed glass, without any selection accord- 
ing to colour; a fair amount of manganese was found to be 
present.—(See analysis A.) It was next thought desirable 
to analyse samples of the almost colourless, and of the 
deeper green coloured fragments, to ascertain whether the 
Manganese has been added in the former in sufficient 
quantity to whiten the glass, or whether it had been made 
from purer materials. 
Colourless glass.—Some of this was in fairly flat pieces, 
like window glass, and had a dull surface somewhat 
resembling that of ground glass; other portions were frag- 
ments of vases or bottles. Although here termed colourless 
for distinction, it shows a pale green tint when viewed 
