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 peculiar gas, a specimen of which I have prepared, and preserved within this inverted glass bell. EMILY. It is a perfectly invisible elastic fluid. MRS. B. Yes; and it may be kept any length of time in this manner over water, as it is not, like the nitric and nitrous acids, absorbable by it. It is rather heavier than atmospherical air, and is incapable of supporting either combustion or respiration. I am going to incline the glass gently on one side, so as to let some of the gas escape-- EMILY. How very curious! --It produces orange fumes like the nitrous acid! that is the more extraordinary, as the gas within the glass is perfectly invisible. MRS. B. It would give me much pleasure if you could make out the reason of this curious change without requiring any further explanation. CAROLINE. 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 glass? 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 quantity 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 glass 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, passes 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.