THE EARTH. 251 



of air n.ixL'd with it ; and the heat of the sun, darting dawn, dis- 

 engages the particles of this air from the grosser tiuid ; the sun's 

 rays being reflected back from the water, carry back with them 

 those bubbles of air and water, which, being lighter than the 

 condensed air, will ascend till they meet with a more rarefied 

 air ; and they will then stand suspended. Experience, however, 

 proves nothing of all this. Particles of air or tire ; are not thus 

 known to ascend with a thin coat of water ; and, in fact, we 

 know that the little particles of steam are solid drops of water. 

 But, besides this, water is known to evaporate more powerfully 

 in the severest frost, than when the air is moderately warm.' 

 Dr Hamilton, therefore, of the university of Dublin, rejecting 

 this t'^eory, has endeavoured to establish another. According 

 to him, as aqua fortis is a menstruum that dissolves iron, and 

 keeps it mixed in the fluid ; as aqua regia is a menstruum that 

 dissolves gold ; or as water dissolves salts to a certain quantity, 

 so air is a menstruum that corrodes and dissolves a certain quan- 

 tity of water, and keeps it suspended above. But however in- 

 genious this may be, it can hardly be admitted : as we know by 

 Mariotte's experiment,^ that if water and air be inclosed toge- 

 ther, instead of the air's acting as a menstruum upon the water, 

 the water will act as a menstruum upon the air, and take it all 

 up. We know also, that of two bodies, that which is most 

 fluid and penetrating, is most likely to be the menstruum of 

 the other ; but water is more fluid and penetrating than air, 

 and therefore the most likely of the two to be the menstruum. 

 We know that all bodies are more speedily acted upon, the 

 more their pnrts are brought into contact with the men- 

 struum that dissolves them ; but water inclosed with com- 

 pressed air, is not the more diminished thereby.* In short, 

 we know, that cold, which diminishes the force of other 

 menstruums, is often found to promote evaporation. In this 

 variety of opinion and uncertainty of conjecture, I cannot avoid 

 thinking that a theory of evaporation may be formed upon very 

 simple and obvious principles, and embarrassed, as far as I can 

 conceive, with very few objecti<jns. 



We know that a repelling power prevails i-n nature, not le*» 



2 Metnoires de I'Academie des Sciences, an. 1705. 



3 Mariiitte, de la Nature de I'Air, p. 97, lOG. 



4 See Boyle's Works, vol. ii. p. Gl'J. 



