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. So it has, indeed! And do the other alkalies produce a similar effect? MRS. B. Exactly the same. --We may now proceed to SODA, which, however important, will detain us but a very short time; as in all its general properties it very strongly resembles potash; indeed, so great is their similitude, that they have been long confounded, and they can now scarcely be distinguished, except by the difference of the salts which they form with acids. The great source of this alkali is the sea, where, combined with a peculiar acid, it forms the salt with which the waters of the ocean are so strongly impregnated. EMILY. Is not that the common table salt? MRS. B. The very same; but again we must postpone entering into the particulars of this interesting combination, till we treat of the neutral salts. Soda may be obtained from common salt; but the easiest and most usual method of procuring it is by the combustion of marine plants, an operation perfectly analogous to that by which potash is obtained from vegetables. EMILY. From what does soda derive its name? MRS. B. From a plant called by us _soda_, and by the Arabs _kali_, which affords it in great abundance. Kali has, indeed, given its name to the alkalies in general. CAROLINE. Does soda form glass and soap in the same manner as potash? MRS. B.