It was the prevailing opinion; and the difference of gas or vapour was thought to depend on the different manner in which caloric was united with the basis of these two kinds of elastic fluids. In vapour, it was considered as in a latent state; in gas, it was said to be chemically combined. But the late researches of Sir H. Davy have given rise to a new theory respecting gasses; and there is now reason to believe that these bodies owe their permanently elastic state, not solely to caloric, but likewise to the prevalence of either the one or the other of the two electricities. EMILY. When you speak, then, of the simple bodies oxygen and nitrogen, you mean to express those substances which are the basis of the two gasses? MRS. B. Yes, in strict propriety, for they can properly be called gasses only when brought to an aeriform state. CAROLINE. In what proportions are they combined in the atmosphere? MRS. B. The oxygen gas constitutes a little more than one-fifth, and the nitrogen gas a little less than four-fifths. When separated, they are found to possess qualities totally different from each other. For oxygen gas is essential both to respiration and combustion, while neither of these processes can be performed in nitrogen gas. CAROLINE. But if nitrogen gas is unfit for respiration, how does it happen that the large proportion of it which enters into the composition of the atmosphere is not a great impediment to breathing? MRS. B. We should breathe more freely than our lungs could bear, if we respired oxygen gas alone. The nitrogen is no impediment to respiration, and probably, on the contrary, answers some useful purpose, though we do not know in what manner it acts in that process. EMILY. And by what means can the two gasses, which compose the atmospheric air, be separated? MRS. B. There are many ways of analysing the atmosphere: the two gasses may be separated first by combustion. EMILY. You surprise me! how is it possible that combustion should separate them? MRS. B. I should previously remind you that oxygen is supposed to be the only simple body naturally combined with negative electricity. In all the other elements the positive electricity prevails, and they have consequently, all of them, an attraction for oxygen.* [Footnote *: If chlorine or oxymuriatic gas be a simple body, according to Sir H. Davy’s view of the subject, it must be considered as an exception to this statement; but this subject cannot be discussed till the properties and nature of chlorine come under examination.] CAROLINE. Oxygen the only negatively electrified body! that surprises me extremely; how then are the combinations of the other bodies performed, if, according to your explanation of chemical attraction, bodies are supposed only to combine in virtue of their opposite states of electricity? MRS. B.