51�Թ�

It is manifest that if different metallic oxides be used with the same glass, mixed tints will be produced, so that by mingling small quantities of oxide of cobalt and protoxide of copper, a blue glass having a greenish hue may be obtained.

Oxide of iron colours glass either green or yellow, according to the nature of the oxide; the silicate of the protoxide of iron being green, and that of the peroxide, yellow of a slightly brownish tint.

Copper forms two oxides, the suboxide and the protoxide; the suboxide colours glass red, while the protoxide renders it green.

Arsenious acid also acts as an oxidizing agent, in that it gives up its oxygen to the protoxide of iron, converting it into the peroxide; but the arsenic itself, which has lost its oxygen, is reduced to the metallic state, and being volatile, does not remain with the glass, but passes off by the flues of the furnace.

There are two oxides of iron: one, the protoxide, which imparts a green colour to glass; and the other the peroxide, whose staining property is yellow.