It seems, by the colour and smell, as if it were converted into nitrous acid gas: yet that cannot be, unless it combines with more oxygen; and how can it obtain oxygen the very instant it escapes from the gla.s.s?

EMILY.

From the atmosphere, no doubt. Is it not so, Mrs. B.?

MRS. B.

You have guessed it; as soon as it comes in contact with the atmosphere, it absorbs from it the additional quant.i.ty of oxygen necessary to convert it into nitrous acid gas. And, if I now remove the bottle entirely from the water, so as to bring at once the whole of the gas into contact with the atmosphere, this conversion will appear still more striking--

EMILY.

Look, Caroline, the whole capacity of the bottle is instantly tinged of an orange colour!

MRS. B.

Thus, you see, it is the most easy process imaginable to convert _nitrous oxyd gas_ into _nitrous acid gas_. The property of attracting oxygen from the atmosphere, without any elevation of temperature, has occasioned this gaseous oxyd being used as a test for ascertaining the degree of purity of the atmosphere. I am going to show you how it is applied to this purpose. --You see this graduated gla.s.s tube, which is closed at one end, (PLATE X. Fig. 2.) --I first fill it with water, and then introduce a certain measure of nitrous gas, which, not being absorbable by water, pa.s.ses through it, and occupies the upper part of the tube. I must now add rather above two-thirds of oxygen gas, which will just be sufficient to convert the nitrous oxyd gas into nitrous acid gas.

CAROLINE.

So it has! --I saw it turn of an orange colour; but it immediately afterwards disappeared entirely, and the water, you see, has risen, and almost filled the tube.

MRS. B.

That is because the acid gas is absorbable by water, and in proportion as the gas impregnates the water, the latter rises in the tube. When the oxygen gas is very pure, and the required proportion of nitrous oxyd gas very exact, the whole is absorbed by the water; but if any other gas be mixed with the oxygen, instead of combining with the nitrous oxygen, it will remain and occupy the upper part of the tube; or, if the gases be not in the due proportion, there will be a residue of that which predominates. --Before we leave this subject, I must not forget to remark that nitrous acid may be formed by dissolving nitrous oxyd gas in nitric acid. This solution may be effected simply by making bubbles of nitrous oxyd gas pa.s.s through nitric acid.

EMILY.

That is to say, that nitrogen at its highest degree of oxygenation, being mixed with nitrogen at its lowest degree of oxygenation, will produce a kind of intermediate substance, which is nitrous acid.

MRS. B.

You have stated the fact with great precision. --There are various other methods of preparing nitrous oxyd, and of obtaining it from compound bodies; but it is not necessary to enter into these particulars. It remains for me only to mention another curious modification of oxygenated nitrogen, which has been distinguished by the name of _gaseous oxyd of nitrogen_. It is but lately that this gas has been accurately examined, and its properties have been investigated chiefly by Sir H. Davy. It has obtained also the name of _exhilarating_ gas, from the very singular property which that gentleman has discovered in it, of elevating the animal spirits, when inhaled into the lungs, to a degree sometimes resembling delirium or intoxication.

CAROLINE.

Is it respirable, then?

MRS. B.

It can scarcely be called respirable, as it would not support life for any length of time; but it may be breathed for a few moments without any other effects, than the singular exhilaration of spirits I have just mentioned. It affects different people, however, in a very different manner. Some become violent, even outrageous: others experience a languor, attended with faintness; but most agree in opinion, that the sensations it excites are extremely pleasant.

CAROLINE.

I think I should like to try it--how do you breathe it?

MRS. B.

By collecting the gas in a bladder, to which a short tube with a stop-c.o.c.k is adapted; this is applied to the mouth with one hand, whilst the nostrils are kept closed with the other, that the common air may have no access. You then alternately inspire, and expire the gas, till you perceive its effects. But I cannot consent to your making the experiment; for the nerves are sometimes unpleasantly affected by it, and I would not run any risk of that kind.

EMILY.

I should like, at least, to see somebody breathe it; but pray by what means is this curious gas obtained?

MRS. B.

It is procured from _nitrat of ammonia_, an artificial salt which yields this gas on the application of a gentle heat. I have put some of the salt into a retort, and by the aid of a lamp the gas will be extricated.--

CAROLINE.

Bubbles of air begin to escape through the neck of the retort into the water apparatus; will you not collect them?

MRS. B.

The gas that first comes over need not be preserved, as it consists of little more than the common air that was in the retort; besides, there is always in this experiment a quant.i.ty of watery vapour which must come away before the nitrous oxyd appears.

EMILY.

Watery vapour! Whence does that proceed? There is no water in nitrat of ammonia?

MRS. B.

You must recollect that there is in every salt a quant.i.ty of water of crystallisation, which may be evaporated by heat alone. But, besides this, water is actually generated in this experiment, as you will see presently. First tell me, what are the const.i.tuent parts of nitrat of ammonia?

EMILY.

Ammonia, and nitric acid: this salt, therefore, contains three different elements, nitrogen and hydrogen, which produce the ammonia; and oxygen, which, with nitrogen, forms the acid.

MRS. B.

Well then, in this process the ammonia is decomposed; the hydrogen quits the nitrogen to combine with some of the oxygen of the nitric acid, and forms with it the watery vapour which is now coming over. When that is effected, what will you expect to find?

EMILY.

Nitrous acid instead of nitric acid, and nitrogen instead of ammonia.

MRS. B.

Exactly so; and the nitrous acid and nitrogen combine, and form the gaseous oxyd of nitrogen, in which the proportion of oxygen is 37 parts to 63 of nitrogen.

You may have observed, that for a little while no bubbles of air have come over, and we have perceived only a stream of vapour condensing as it issued into the water. --Now bubbles of air again make their appearance, and I imagine that by this time all the watery vapour is come away, and that we may begin to collect the gas. We may try whether it is pure, by filling a phial with it, and plunging a taper into it--yes, it will do now, for the taper burns brighter than in the common air, and with a greenish flame.

CAROLINE.

But how is that? I thought no gas would support combustion but oxygen or chlorine.

MRS. B.

Or any gas that contains oxygen, and is ready to yield it, which is the case with this in a considerable degree; it is not, therefore, surprising that it should accelerate the combustion of the taper.