We have mentioned the _fixed alkalies_ in a preceding section. We may add here, that by the discoveries of Sir Humphry Davy, in the year 1807, the base of caustic, or pure _potash_, is now known to consist of a light, white metallic substance, to which the name of _potassium_ has been given; it is of the consistence of soft wax; at a freezing temperature it is hard, brittle, and solid; when thrown upon water it instantly takes fire, hydrogen gas escapes, and an _oxide of potassium_, or caustic pot-ash, is produced. The potash and pearl-ash of the shops we must not forget, are combinations of _carbonic acid_ and pot-ash, hence they effervesce with all the acids; but caustic pot-ash, containing no carbonic acid, combines with any of the acids without effervescence. The SODA, as obtained from barilla, is a carbonate of soda; pure soda, or caustic soda, was, till the discoveries of Sir Humphry Davy, supposed to be, as well as potash, a simple substance. It is now, however, known to consist of a metallic substance of the colour of lead, but, nevertheless, lighter than water; upon which, when thrown, it produces, like potassium, violent action, yet does not, in general, like potassium, inflame. It is called _sodium_; pure soda consists therefore of sodium and oxygen, hence it is an _oxide of sodium_. These discoveries of the composition of the fixed alkalies are of infinite importance in the arts. The alkalies contain some very striking properties: _Their taste is acrid, burning and urinous. They generally change the blue colours of vegetable infusions green. When mixed with silex or flint, by exposure to great heat they form glass, and they render oils miscible with water, and hence combine with them forming soaps. They effervesce_ (_when combined with carbonic acid_,) with many other acids, and form neutral salts with all the acids. The _volatile alkali_ or _ammonia_, on exposure to air, flies entirely away. Pot-ash, either in its caustic state, or in that of a carbonate, absorbs moisture from the air, and liquifies. While soda, on the contrary, and many of its combinations, effloresce in the air; they, nevertheless, effervesce, and combine with the acids in a similar way to pot-ash. We have mentioned how pot-ash is obtained in a preceding section. Soda is commonly procured from the ashes of marine plants; the _barilla_ of commerce is obtained, it is said, in Spain, chiefly from many species of the _salsola_, or salt-wort. Barilla is an impure subcarbonate of soda, it is used largely in the manufacture of soap. We now proceed to notice the nature of _acids_.