224 PHILOSOPHICAX TRANSACTIONS. [aNNO 1765, 



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 superficies be increased. Sixteen leaden bullets, of an ounce each, weigh as 

 much in water as one of a pound, whose superficies is less. Therefore the sup- 

 porting of salt in water is not owing to its superficies being increased. A lump 

 - of salt, though laid at rest at the bottom of a vessel of water, will dissolve in it, 

 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 par- 

 ticles 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, com- 

 municates its saltness till all is equal, and the sooner if there is a little motion of 

 . the water. Even earth will dissolve, or mix with air. A stroke of a horse's 

 hoof on the ground in a hot dusty road, will raise a cloud of dust, that shall, if 

 there be a light breeze, expand every way till perhaps near as large as a common 

 house. It is not by mechanical motion communicated to the particles of dust bv 

 the hoof that they fly so far, nor by the wind that they spread so wide. But 

 the air near the ground, more heated by the hot dust struck into it, is rarefied 

 and rises, and in rising mixes with the cooler air, and communicates of its dust 

 to it, and it is at length so difflised as to become invisible. Quantities of dust 

 are thus carried vip in dry seasons. Showers wash it from the air and bring it 

 down again. For water attracting it stronger, it quits the air and adheres to 

 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 there- 

 fore be in continual motion, A small quantity of fire mixed with water, or de- 

 gree of heat in it, so weakens the cohesion of its particles, that those on the 

 surface easily quit it, and adhere to the particles of air, A greater degree of heat 

 is required to break the cohesion between water and 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 cohe- 

 sions, that would obstruct, refract, or reflect the light. Hence, when we 

 breathe in warm air, though the same quantity of moisture may be taken up 



