The general properties of alkalies are, an acrid burning taste, a pungent smell, and a caustic action on the skin and flesh. CAROLINE. I wonder they should be caustic, Mrs. B., since they contain so little oxygen. MRS. B. Whatever substance has an affinity for any one of the constituents of animal matter, sufficiently powerful to decompose it, is entitled to the appellation of caustic. The alkalies, in their pure state, have a very strong attraction for water, for hydrogen, and for carbon, which, you know, are the constituent principles of oil, and it is chiefly by absorbing these substances from animal matter that they effect its decomposition; for, when diluted with a sufficient quantity of water, or combined with any oily substance, they lose their causticity. But, to return to the general properties of alkalies--they change, as we have already seen, the colour of syrup of violets, and other blue vegetable infusions, to green; and have, in general, a very great tendency to unite with acids, although the respective qualities of these two classes of bodies form a remarkable contrast. We shall examine the result of the combination of acids and alkalies more particularly hereafter. It will be sufficient at present to inform you, that whenever acids are brought in contact with alkalies, or alkaline earths, they unite with a remarkable eagerness, and form compounds perfectly different from either of their constituents; these bodies are called _neutral_ or _compound salts_. The dry white powder which you see in this phial is pure caustic POTASH; it is very difficult to preserve it in this state, as it attracts, with extreme avidity, the moisture from the atmosphere, and if the air were not perfectly excluded, it would, in a very short time, be actually melted. EMILY. It is then, I suppose, always found in a liquid state? MRS. B. No; it exists in nature in a great variety of forms and combinations, but is never found in its pure separate state; it is combined with carbonic acid, with which it exists in every part of the vegetable kingdom, and is most commonly obtained from the ashes of vegetables, which are the residue that remains after all the other parts have been volatilised by combustion. CAROLINE. But you once said, that after all the volatile parts of a vegetable were evaporated, the substance that remained was charcoal? MRS. B. I am surprised that you should still confound the processes of volatilisation and combustion. In order to procure charcoal, we evaporate such parts as can be reduced to vapour by the operation of heat alone; but when we _burn_ the vegetable, we burn the carbon also, and convert it into carbonic acid gas. CAROLINE. That is true; I hope I shall make no more mistakes in my favourite theory of combustion. MRS. B.