VOL. XXXVI.] PHILOSOPHICAL TKANSACTIONS. 32/ 



air, why do not those clouds always expand when the ambient air is rarefied, 

 and presses less than it did before ; and also suffer a condensation, as the am- 

 bient air is condensed by the accumulation of the superior air ? 



If this condensation and rarefaction should happen to the clouds, they would 

 always continue at the same height, contrary to observation; and we should 

 never have any rain. 



From all this it follows, that the condensation and rarefaction of the vapours, 

 which make clouds, must depend on another principle than the condensation 

 and rarefaction of the air : and that there is such a principle, the Dr. endeavours 

 to show. 



Lemma. The Particles of all Fluids have a repellent force. — Fluids are elastic 

 or unelastic: the elastic fluids have their density proportionable to their com- 

 pression ; and Sir Isaac Newton has demonstrated (Princip. lib. 1, sect. 5) that 

 they consist of parts that repel each other from their respective centres. Un- 

 elastic fluids, like mercury, water, and other liquors, are by experiments found 

 to be incompressible ; for water in the Florentine experiment could not by any 

 force be compressed into less room, but oozed like dew through the pores of 

 the hollow golden ball in which it was confined, when a force was applied to 

 press the ball out of its spherical, into a less capacious figure. Now this pro- 

 perty of water, and other liquors, must be entirely owing to the centrifugal 

 force of its parts, and not its want of vacuity ; since salts may be imbibed by 

 water without increasing its bulk, as appears by the increase of its specific 

 gravity. So metals, which (singly) have a certain specific gravity beyond 

 which they cannot be condensed, will yet receive each other in their interstices, 

 so as to make a compound specifically heavier than the heaviest of them ; as is 

 experienced in the mixture of copper and tin. 



Scholium. — By increasing the repellent force of the particles, an unelastic or 

 incompressible fluid may become elastic, or a solid, at least a great part of it 

 may be changed into an elastic fluid; and, vice versa, by diminishing the repel- 

 lent force, an elastic fluid may be reduced to an unelastic one, or to a solid. 

 That the particles of quicksilver, water, and other liquors, are likewise endued 

 with an attractive force, is evident from those substances running into drops in 

 an exhausted receiver, as well as in the air, and likewise their adhering to other 

 bodies. The attraction and repulsion exert their forces differently : the attrac- 

 tion only acts on the particles, which are in contact, or very near it: in which 

 case it overcomes the repulsion so far as to render that fluid unelastic, which 

 otherwise would be so; but it does not wholly destroy the repulsion of the parts 

 of the fluid, because it is on account of that repulsion that the fluid is then 

 incompressible. When by heat or fermentation the particles are separated from 



