I think this will sufficiently shew the Existence of this Fire in the Air; and, if any Regard be had to what I think its Power and Use is in the World, that it will intrude itself and force its Way into any Thing where less of it is, and so join itself to it by being in a greater Quantity; as has been shewn by many electrical Experiments. You may suppose a Person sitting, as it is too frequently found they are, near a Door, or in a Window, when they are in a warm Temperature, and in Perspiration; if you believe that there can be any Probability in the Conjecture I have offer’d to your Consideration, is it not natural for any of this Fire, which passes as frequently through the Air in the Daytime (though unobserved) as when it is seen in the Night; I say, Why is it not natural for it to force its Entrance into any Person or Thing? especially as it comes then with the Assistance of the Stream of Air the Person sits in, and with which it is driven. In order to make this Mischief the more to be regarded, I will endeavour to shew the natural State of the Air itself. Many Writers about it chuse to divide it into two Sorts; the first is the pure _Æther_, which is supposed to be moving above our Atmosphere; the second is the common Air, which is supposed to be within our Atmosphere. I confess, the Feats attributed to the mighty Weight of our Atmosphere, in causing Siphons and Pumps, _&c._ to operate, I never could understand; but if I were to account for their Operations, as well as that of a Barometer, by the Elasticity of the Air, I think I could more easily and more naturally shew it. Notwithstanding what has been advanced concerning the _Æther_, which is believed to inhabit above our Atmosphere, I chuse rather to suppose, that the Air is an Element as well as Fire, and that the Difference in it is only betwixt heavy and foul Air, and clean and light Air. That which comes on the highest Mountains is clean, and free from our Fogs and Putrefactions, and, consequently, more elastic. As a Proof of this, I would recommend the following Experiment: Fill a Bladder with this clean Air; then press it with a Weight just sufficient to make it give way; and you will find, that, by reason of its Elasticity, it will yield much further, than if it were fill’d with the other Air, which is impregnated with foggy and aqueous Particles. Now if, as in a Barometer, the Quicksilver is suspended by the Air on the Top of the Tube, which was extracted or emerged out of the Quicksilver, by the Weight of the said Quicksilver, and as that Air in the Barometer cannot but have a Communication with the ambient Air, the Air within the Barometer must thence be affected, by its becoming less elastic also.