You must observe, that whenever oxygen is taken from the atmosphere, it is by decomposing the oxygen gas; we cannot do the same with the nitrogen gas, because nitrogen has a stronger affinity for caloric than for any other known principle: it appears impossible therefore to separate it from the atmosphere by the power of affinities. But if we cannot obtain the oxygen gas, by this means, in its separate state, we have no difficulty (as you have seen) to procure it in its gaseous form, by taking it from those substances that have absorbed it from the atmosphere, as we did with the oxyd of manganese. EMILY. Can atmospherical air be recomposed, by mixing due proportions of oxygen and nitrogen gases? MRS. B. Yes: if about one part of oxygen gas be mixed with about four parts of nitrogen gas, atmospherical air is produced.* [Footnote *: The proportion of oxygen in the atmosphere varies from 21 to 22 per cent.] EMILY. The air, then, must be an oxyd of nitrogen? MRS. B. No, my dear; for there must be a chemical combination between oxygen and nitrogen in order to produce an oxyd; whilst in the atmosphere these two substances are separately combined with caloric, forming two distinct gases, which are simply mixed in the formation of the atmosphere. I shall say nothing more of oxygen and nitrogen at present, as we shall continually have occasion to refer to them in our future conversations. They are both very abundant in nature; nitrogen is the most plentiful in the atmosphere, and exists also in all animal substances; oxygen forms a constituent part, both of the animal and vegetable kingdoms, from which it may be obtained by a variety of chemical means. But it is now time to conclude our lesson. I am afraid you have learnt more to-day than you will be able to remember. CAROLINE. I assure you that I have been too much interested in it, ever to forget it. In regard to nitrogen there seems to be but little to remember; it makes a very insignificant figure in comparison to oxygen, although it composes a much larger portion of the atmosphere. MRS. B. Perhaps this insignificance you complain of may arise from the compound nature of nitrogen, for though I have hitherto considered it as a simple body, because it is not known in any natural process to be decomposed, yet from some experiments of Sir H. Davy, there appears to be reason for suspecting that nitrogen is a compound body, as we shall see afterwards. But even in its simple state, it will not appear so insignificant when you are better acquainted with it; for though it seems to perform but a passive part in the atmosphere, and has no very striking properties, when considered in its separate state, yet you will see by-and-bye what a very important agent it becomes, when combined with other bodies. But no more of this at present; we must reserve it for its proper place. CONVERSATION VII. ON HYDROGEN. CAROLINE.