To the three materials of Flint Glass—viz., sand, red lead, and carbonate of potash, is added a small quantity of saltpetre, which increases the dose of oxygen, and assists to drive off the globules of air in the liquid Glass; there are, also, added to each pot a few ounces of crystallized manganese, the effects of which will be described hereafter. The whole is carefully mixed together, and sifted through a coarse sieve: in this state it is called batch, or frit, and is of a salmon colour. It is by the purity of these materials that the British manufacturer is enabled to maintain ascendancy over the foreigner for pure metal or Flint Glass; and its superior quality is especially due to the excellence of the red lead, which imparts density, refractibility, transparency, and pellucid brilliancy.

If Flint Glass be required of great density for optical purposes, it is only to add a larger dose of red lead, or litharge, beyond what might be termed the atomic mixture of one of alkali, two of lead, and three of sand, (of the specific gravity of 3.200,) and it will produce Glass of higher density, but less permanent, in the ratio of its increased specific gravity. Were the proportion of lead twice the atomic quantity, the surface of the Glass would be more or less liable to decompose by exposure to the atmosphere, and need constant wiping to preserve its transparency; it might even require to be repolished for the same object, and that repeatedly. Excess of alkali is equally as injurious as excess of lead, and, indeed, more destructive, through the effects of time and atmospheric influence, as will be more fully explained in a subsequent part of this work. Crystallized black oxide of manganese is best fitted for a Glass-maker's purpose, if well washed from aluminous earth, with which it is ordinarily found combined in mines. The most