Air in this manner encompasses, is in contact with, confines, and compresses all bodies. It insinuates itself into their penetrable passages, exerts all its power either on solids, or fluids, and finding in bodies some elements to which it has a tendency, unites with them. By its weight and perpetual motion, it strongly agitates those parts of the bodies in which it is contained, rubs, and intermixes them intimately together. By disuniting some, and joining others, it produces very singular effects, not easily accomplished by any other means.—That this element has such surprising powers, is evident from the following experiment. “Fermentable parts duly prepared and disposed in the vacuum of Mr. Boyle’s air-pump will not ferment, though acted upon by a proper heat; but, discharging their air, remain unchanged.” SECTION III. _OF WATER._ As water is perpetually an object of our senses, and made use of for most of the purposes of life, it might be imagined the nature of this element was perfectly understood: but they who have enquired into it with the greatest care, find it very difficult to form a just idea of it. One reason of this difficulty is, water is not easily separated from other bodies, or other bodies from water. Hartshorn, after having been long dried, resists a file more than iron; yet, on distillation, yields much water. I have already observed, that air is intimately mixed with, and possibly never entirely separated from it, but in a _vacuum_; how is it possible then ever to obtain water perfectly pure? In its most perfect state, we understand it to be a liquor very fluid, inodorous, insipid, pellucid, and colourless, which, in a certain degree of cold, freezes into a brittle, hard, glassy ice. Lightness is reckoned a perfection in water, that which weighs less being in general the purest. Hence the great difficulty of determining the standard weight it should have. Fountain, river, or well waters, by their admixture with saline, earthy, sulphureous, and vitriolic substances, are rendered much heavier than in their natural state; on the other hand, an increase of heat, or an addition of air, by varying the expansion, diminishes the weight of water. A pint of rain-water, supposed to be the purest, is said to weigh 15 ounces, 1 drachm, and 50 grains, but, for the reasons just now mentioned, this must differ in proportion as the seasons of the year do from each other. Another property of water, which it has in common with other liquors, is its fluidity, which is so great, that a very small degree of heat, above the freezing point, makes it evaporate. Experiments to ascertain the proportion steamed away of the quantity of water used in brewing, is an object worthy of the artist’s curiosity; but the purer the water is, the more readily it evaporates. Sea-water, which is supposed to contain one fortieth part of salt, more forcibly resists the power of fire, and wastes much less, than that which is pure.