same time blackened. But the vegetable substance in which the gallic acid most abounds is nutgall, a kind of excrescence that grows on oaks, and from which the acid is commonly obtained for its various purposes.

Mrs. B. We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating of phosphorus, you have seen how these acids may be obtained from it by combustion?

Emily. Yes; but I should be much surprised if it was the usual method of obtaining them, since it is so very difficult to procure phosphorus in its pure state.

Mrs. B. You are right, my dear; the phosphoric acid, for general purposes, is extracted from bones, in which it is contained in the state of phosphat of lime; from this salt the phosphoric acid is separated by means of the sulphuric, which combines with the lime. In its pure state, phosphoric acid is either liquid or solid, according to its degree of concentration.

Amongst the salts formed by this acid, phosphat of lime is the only one that affords much interest; and this, we have already observed, constitutes the basis of all bones. It is also found in very small quantities in some vegetables.

Conversation XV.

Of the nitric and carbonic acids; or the combinations of oxygen with nitrogen and carbone; and of the nitrats and carbonats.

Mrs. B.

I am almost afraid of introducing the subject of the NITRIC ACID, as I am sure that I shall be blamed by Caroline, for not having made her acquainted with it before.

Caroline. Why so, Mrs. B-?

Τ

Mrs. B. Because you have long known its radical, which is nitrogen or azote; and, in treating of that element, I did not even hint that it was the basis of an acid.

Caroline. Indeed, that appears to me a great omission; for you have made us acquainted with all the other acids, in treating of their radicals.

Emily. I would advise you not to be too hasty in your censure, Caroline; for I dare say that Mrs. B. had some very good reason for not mentioning this acid

sooner.

Mrs. B. I do not know whether you will think the reason sufficiently good to acquit me; but the omis sion, I assure you, did not proceed from negligence. You may recollect that nitrogen was one of the first simple bodies which we examined; you were then ignorant of the theory of combustion, which I believe was, for the first time, mentioned in that lesson; and there fore it would have been in vain, at that time, to have attempted to explain the nature and formation of acids.

Caroline. I wonder, however, that it never occur. red to us to inquire whether nitrogen could be acidified; for, as we knew it was classed amongst the combus tible bodies, it was natural to suppose that it might produce an acid..

for

Mrs. B. That is not a necessary consequence; it might combine with oxygen only in the degree requisite to form an oxyd. But you will find that nitrogen is susceptible of various degrees of oxygenation, some of which convert it merely into an oxyd, and others give it all the acid properties.

The acids, resulting from the combination of oxygen with nitrogen, are called the NITROUS and NITRIC acids. We will begin with the NITRIC, in which nitrogen is in the highest state of oxygenation. This acid naturally exists in the form of gas; but it is so extremely soluble in water, and has so great an affinity for it, that one grain of water will absorb and condense ten grains of acid gas, and form the limpid fluid which you see in

this bottle.

Caroline. What a strong offensive smell it has!

Mrs. B. This acid contains a greater abundance of oxygen than any other, but it retains it with very little force.

Emily. 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 organized 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 shews you how easily this acid is decomposed by combustible bo dies, since these effects must depend upon the absorp tion of its oxygen.

Nitric acid has been used in the arts from time im memorial, but it is not more than 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 gasses.

Emily. The nitrogen and oxygen gasses, that compose the atmosphere, 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 extent as to be perceivable.

* The proportions stated by Mr. Davy, in his Chemical Re acarches, are as I to 2. 389.

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 called so? 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 that 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. By the intervention of sulphuric acid, which combines with the potash, and sets the nitric acid at liberty. This I can easily shew you, by mixing some nitrat of potash and sulphuric acid in this retort, and heating it over a lamp; the nitric acid will come over in the form of vapour, which we shall collect in a glass bell. This acid diluted in water is commonly called aqua fortis, if Caroline will allow me to mention that

name.

Caroline. I have often heard that aqua fortis will dissolve almost all metals; it is no doubt because it yields its oxygen so easily.

Mrs. B. Yes; and from this powerful solvent property, it derived the name of aqua fortis, or strong wa ter. Do you not recollect that we oxydated, and afterwards dissolved some copper in this acid?

Emily. If I remember right, the nitrat of copper was the first instance you gave us of a compound salt. Caroline. Can the nitric acid be completely decom. posed and converted into nitrogen and oxygen?

Emily. That cannot be the case, Caroline, since the acid can be decomposed only by the combination of its constituents with other bodies.

Mrs. B. True; but caloric is sufficient for this purpose. By making the acid pass through a red hot por

celain tube, it is decomposed; the nitrogen and oxygen regain the caloric which they had lost in combining, and are thus both restored to their gaseous state. The nitric acid may also be partly decomposed, and is by this means converted into NITROUS ACID.

Caroline. This conversion must be easily effected, as the oxygen is so slightly combined with the nitrogen.

Mrs. B. The partial decomposition of nitric acid is readily effected by most metals; but it is sufficient to expose the nitric acid to a very strong light to make it give out oxygen gas, and be thus converted into nitrous acid. Of this acid there are various degrees, according to the proportions of oxygen which it contains ; the strongest and that into which the nitric acid is first converted, is of a yellow colour, as you see it in this bottle.

Caroline. How it fumes when the stopper is taken

out.

Mrs. B. The acid exists naturally in a gaseous state, and is here so strongly concentrated in water that it is constantly escaping.

Here is another bottle of nitrous acid, which, you see is of an orange red colour; this acid is weaker, the nitrogen being combined with a smaller quantity of oxygen; and with a still less proportion of oxygen it is an olive green colour, as it appears in this third bottle. In short, the weaker the acid, the deeper is its colour. Nitrous acid acts still more powerfully on some inflammable substances than the nitric.

Emily. I am surprised at that, as it contains less oxygen.

Mrs. B. But, on the other hand, it parts with its oxygen much more readily: you may recollect that we once inflamed oil with this acid.

The next combinations of nitrogen and oxygen form only oxyds of nitrogen, the first of which is commonly called nitrous air: or more properly nitric oxyd gas. This may be obtained from nitric acid, by exposing the latter to the action of metals, as in dissolving them it does not yield the whole of its oxygen, but retains a portion of this principle sufficient to convert it into this