Note c — BOOK II.

I took notice before in Book I. Chap. 3. Note (a). That the Vapours constituting Clouds and Rain, are Vesiculæ of Water detached by Heat. The manner of which I conceive to be thus; Heat being of an agile Nature, or the lightest of all Bodies, easily breaks loose from them; and if they are humid, in its Passage, carries along with it Particles, or little Cases of the Water; which being lighter than Air, are buoyed up thereby, and swim in it; until by knocking against one another, or being thickened by the Cold, (as in the Note before-cited,) they are reduced into Clouds and Drops.

Having mentioned the manner how Vapours are raised, and there being more room here than in the Note before-cited, I shall, for the Illustration of Natures Process, take notice of three Things observable to our purpose, in Water over the Fire. 1. That the Evaporations are proportional to the Heat ascending out of the Water. A small Heat throws off but few Vapours, scarce visible: A greater Heat, and ascending in greater Quantities, carries off grosser, larger, and more numerous Vesiculæ, which we call a Steam: And if the Heat breaks through the Water with such a Fury, as to lacerate and lift up great Quantities or Bubbles of Water, too heavy for the Air to carry or buoy up, it causeth what we call Boyling. And the Particles of Water thus mounted up by the Heat, are visible Sphærules of Water, if viewed with a Microscope, as they swim about in a Ray of the Sun let into a dark Room, with warm Water underneath; where some of the Vapours appear large, some smaller Sphærules, according (no doubt) to the larger and lesser Quantities of Heat blowing them up and carrying them off. 2. If these Vapours be intercepted in their Ascent by any Context, especially cold Body, as Glass, Marble, &c. they are thereby reduced into Drops, and Masses of Water, like those of Rain, &c. 3. These Vapours in their Ascent from the Water, may be observed, in cold frosty Weather, either to rise but a little above the Water, and there to hang, or to glide on a little above its Surface: Or if the Weather be very cold, after a little ascent, they may be seen to fall back again into the Water; in their Ascent and Descent describing a Curve somewhat like that of an Arrow from a Bow. But in a warmer Air, and still, the Vapours ascend more nimbly and copiously, mounting up aloft, till they are out of Sight. But if the Air be warm and windy too, the Vapours are sooner carried out of Sight, and make way for others. And accordingly I have often observed, that hot Liquors, if not set too thin, and not frequently stirred, cool slower in the greatest Frosts, than in temperate Weather, especially if windy. And it is manifest by good Experiments, that the Evaporations are less at those times than these; less by far in the Winter than the warmer Months.