_Physical and Meteorological Observations, Conjectures, and Suppositions._--Read at the Royal Society, June 3, 1756.
The particles of air are kept at a distance from each other by their mutual repulsion * *
Whatever particles of other matter (not endued with that repellancy) are supported in air, must adhere to the particles of air, and be supported by them; for in the vacancies there is nothing they can rest on.
Air and water mutually attract each other. Hence water will dissolve in air, as salt in water.
The specific gravity of matter is not altered by dividing the matter, though the superfices be increased. Sixteen leaden bullets, of an ounce each, weigh as much in water as one of a pound, whose superfices is less.
Therefore the supporting of salt in water is not owing to its superfices being increased.
A lump of salt, though laid at rest at the bottom of a vessel of water, will dissolve therein, and its parts move every way, till equally diffused in the water; therefore there is a mutual attraction between water and salt. Every particle of water assumes as many of salt as can adhere to it; when more is added, it precipitates, and will not remain suspended.
Water, in the same manner, will dissolve in air, every particle of air assuming one or more particles of water. When too much is added, it precipitates in rain.
But there not being the same contiguity between the particles of air as of water, the solution of water in air is not carried on without a motion of the air so as to cause a fresh accession of dry particles.
Part of a fluid, having more of what it dissolves, will communicate to other parts that have less. Thus very salt water, coming in contact with fresh, communicates its saltness till all is equal, and the sooner if there is a little motion of the water. * *
*Air, suffering continual changes in the degrees of its heat, from various causes and circumstances, and, consequently, changes in its specific gravity, must therefore be in continual motion.
A small quantity of fire mixed with water (or degree of heat therein) so weakens the cohesion of its particles, that those on the surface easily quit it and adhere to the particles of air.
Air moderately heated will support a greater quantity of water invisibly than cold air; for its particles being by heat repelled to a greater distance from each other, thereby more easily keep the particles of water that are annexed to them from running into cohesions that would obstruct, refract, or reflect the light.