CHAP. III.
Wherein, from the Experiments in the foregoing Chapter, is shewed, the Cause of the Motion of the Mercury in the BAROMETER.
OR the doing of this, it will first be acknowledg’d, That not only several sorts of Sulphur, but also of Volatile Salts, are continually sublimed from most Bodies into the Aer. So Lightning, from the celerity of the ascension, appears to be made of a Meteor, which is Nitro-Sulphureous. Snow dependeth upon a Mixture of Nitrous, and other Salts; as is evident, from the regularly and differently Figur’d Parts, which compose the whole Body of a Snowy Cloud, before it clusters into Flakes. And one reason, why Rain is the best Water for any Soyl, is because it is impregnated with divers Volatile and Fruitful Salts. And so from other Meteors.
2. §. And next, that these Salts, are not always in the same Quantity, Proportion, and State, in the Aer: but that sometimes they are more copious; at others, less: sometimes, one more copious, than an other: sometimes, more plentifully dissolved; at others, more sparingly: and that, either as they are more or less pure and dissoluble; or according to the quantity of the Vaporous Parts in the Aer, in which they are incorporated or dissolved.
3. §. Thus much being granted, from the Experiments in the foregoing Chapter compared together, we may resolve our selves about some Phænomena in the Barometre. Which seems to vary, not so much with the meer Weight of the Aer, which hitherto hath been supposed: as by the different pressure it makes, in being crowded more at one time, than at another. That is, according as certain Nitrous, or other Saline Bodies, take up less Space in the Aer, when dissolved in the Watery Parts therein, than while they are undissolved.
4. §. And therefore it is especially to be observed, That as the Mercury commonly riseth in the Cylinder for some days, but always for some time, before the change of the Weather, whether for Snow or Rain: So, that then it presently falleth again, even before the Snow or Rain falls. Whereas, if the Weight of the Aer, were the only, or the chief Cause of the ascent of the Mercury; than as it riseth all the while the Weather is gathering, so it would keep its standing or heighth, until the Weather breaks and falleth down: which yet it never doth, but always falls before it; sometimes no less than a whole day. The Cause whereof is, in that all the while the Mercury riseth in the Cylinder, the Aer is crowded with more and more Saline Parts, which by the Winds, or otherwise, are carryed into it; and so causeth it to press upon the Mercury in the Box: but after that in some time the Salts are dissolved or incorporated in the Aqueous Parts of the Aer, as in Rain or Snow; so soon as that is done, there is some Space gaind; and so, before any Weather falleth, the Aer is less crowded, and presseth less upon the Mercury in the Box, which gives way to its descent in the Cylinder.
5. §. From hence also it is, that the Mercury riseth higher with Cold Winds, than it doth with those which are Warme. Both because that in cold Winds there is the greatest quantity of Nitre: and that the coldest Winds, are usually the dryest. So that the Nitre wanting Moysture fully to dissolve it; it takes up so much the greater space, and so causeth a greater pressure in the Aer, as hath been said.
6. §. Lastly, For the same reason it comes to pass, that the Mercury first riseth higher, and then falleth lower before Snow, than it doth before Rain. Because that for the production of Snow, the Aer is crowded with a greater quantity of Nitre, or some other like Salts; which before they are dissolved, take up so much the more space; and afterwards so much the less, even before the Snow falls: as hath been proved.
FINIS.