Then it must be a powerful caustic, both from the facility with which it parts with its oxygen, and the quantity which it affords? MRS. B. Very well, Emily; both cause and effect are exactly such as you describe: nitric acid burns and destroys all kinds of organised matter. It even sets fire to some of the most combustible substances. --We shall pour a little of it over this piece of dry warm charcoal--you see it inflames it immediately; it would do the same with oil of turpentine, phosphorus, and several other very combustible bodies. This shows you how easily this acid is decomposed by combustible bodies, since these effects must depend upon the absorption of its oxygen. Nitric acid has been used in the arts from time immemorial, but it is only within these twenty-five years that its chemical nature has been ascertained. The celebrated Mr. Cavendish discovered that it consisted of about 10 parts of nitrogen and 25 of oxygen.* These principles, in their gaseous state, combine at a high temperature; and this may be effected by repeatedly passing the electrical spark through a mixture of the two gases. [Footnote *: The proportion stated by Sir H. Davy, in his Chemical Researches, is as 1 to 2.389.] EMILY. The nitrogen and oxygen gases, of which the atmosphere is composed, do not combine, I suppose, because their temperature is not sufficiently elevated? CAROLINE. But in a thunder-storm, when the lightning repeatedly passes through them, may it not produce nitric acid? We should be in a strange situation, if a violent storm should at once convert the atmosphere into nitric acid. MRS. B. There is no danger of it, my dear; the lightning can affect but a very small portion of the atmosphere, and though it were occasionally to produce a little nitric acid, yet this never could happen to such an extent as to be perceivable. EMILY. But how could the nitric acid be known, and used, before the method of combining its constituents was discovered? MRS. B. Before that period the nitric acid was obtained, and it is indeed still extracted, for the common purposes of art, from the compound salt which it forms with potash, commonly called _nitre_. CAROLINE. Why is it so called? Pray, Mrs. B., let these old unmeaning names be entirely given up, by us at least; and let us call this salt _nitrat of potash_. MRS. B. With all my heart; but it is necessary that I should, at least, mention the old names, and more especially those which are yet in common use; otherwise, when you meet with them, you would not be able to understand their meaning. EMILY. And how is the acid obtained from this salt? MRS. B.