e perfect glass; for if the
sand, or flint, from which the siliceous earth is obtained, be mixed
with any metallic particles, or other substance, which cannot be
vitrified, the glass will be discoloured, or defaced, by opake specks.

CAROLINE.

That, I suppose, is the reason why objects so often appear irregular and
shapeless through a common glass-window.

MRS. B.

This species of imperfection proceeds, I believe, from another cause. It
is extremely difficult to prevent the lower part of the vessels, in
which the materials of glass are fused, from containing a more dense
vitreous matter than the upper, on account of the heavier ingredients
falling to the bottom. When this happens, it occasions the appearance of
veins or waves in the glass, from the difference of density in its
several parts, which produces an irregular refraction of the rays of
light that pass through it.

Another species of imperfection sometimes arises from the fusion not
being continued for a length of time sufficient to combine the two
ingredients completely, or from the due proportion of potash and silex
(which are as two to one) not being carefully observed; the glass, in
those cases, will be liable to alteration from the action of the air, of
salts, and especially of acids, which will effect its decomposition by
combining with the potash, and forming compound salts.

EMILY.

What an extremely useful substance potash is!

MRS. B.

Besides the great importance of potash in the manufactures of glass and
soap, it is of very considerable utility in many of the other arts, and
in its combinations with several acids, particularly the nitric, with
which it forms saltpetre.

CAROLINE.

Then saltpetre must be a _nitrat of potash_? But we are not yet
acquainted with the nitric acid?

MRS. B.

We shall therefore defer entering into the particulars of these
combinations till we come to a general review of the compound salts. In
order to avoid confusion, it will be better at present to confine
ourselves to the alkalies.

EMILY.

Cannot you show us the change of colour which you said the alkalies
produced on blue vegetable infusions?

MRS. B.

Yes; very easily. I shall dip a piece of white paper into this syrup of
violets, which, you see, is of a deep blue, and dyes the paper of the
same colour. --As soon as it is dry, we shall dip it into a solution of
potash, which, though itself colourless, will turn the paper green--

CAROLINE.