145 A HISTORY OF 



But however ingenious this may be, it can hardly be admitted ; as we 

 know by Mariotte's experiment,* that if water and air be inclosed to- 

 gether, 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. Wt- 

 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 act- 

 ed upon, the more their parts are brought into contact with the men- 

 struum that dissolves them ; but water, inclosed with compressed 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 objections. 



We know that a repelling power prevails in nature, not less than 

 an attractive one. This repulsion prevails strongly between the body 

 of fire and that of water. If I plunge the end of a red-hot bar of iron 

 into a vessel of water, the fluid rises, and large drops of it fly up i-i 

 all manner of directions, every part bubbling and steaming until the 

 iron be cold. Why may we not for a moment compare the rays of 

 the sun, darted directly upon the surface of the water, to so many 

 bars of red-hot iron, each bar indeed infinitely small, but not the less 

 powerful ? In this case, wherever a ray of fire darts, the water, from 

 its repulsive quality, will be driven on all sides ; and, of consequence, 

 as in the case of the bar of iron, a part of it will rise. The parts 

 thus rising, however, will be extremely small ; as the ray that darts is 

 extremely so. The assemblage of the rays darting upon the water in 

 this manner, will cause it to rise in a light thin steam above the sur- 

 face ; and as the parts of this steam are extremely minute, they will 

 be lighter than air, and consequently float upon it. There is no need 

 for supposing them bubbles of water filled with fire ; for any substance, 

 even gold itself, will float on air, if its parts be made small enough ; or, 

 in other words, if its surface be sufficiently increased. This water, thus 

 disengaged from the general mass, will be still farther attenuated and 

 broken by the reflected rays, and, consequently, more adapted foi 

 ascending. 



From this plain account, every appearance in evaporation may be 

 easily deduced. The quantity of heat increases evaporation, because 

 it raises a greater quantity of stearn. The quantity of wind increases 

 evaporation ; for, by waving the surface of the water, it thus exposes 

 a greater surface to the evaporating rays. A dry frost, in some mea- 

 sure, assists the quantity of evaporation ; as the quantity of rays are 

 found to be no way diminished thereby. Moist weather alone pre- 

 vents evaporation ; for the rays being absorbed, refracted, and broken, 

 by the intervening moisture, before they arrive at the surface, cannot 

 produce the effect ; and the vapour will rise in a small proportion. 



Thus far we nav* accounted for the ascent of vapours ; but to ac- 



Mariotte, tie Id Nature de 1'Air, p. 97, 106. \ See Boyle'8 Works, vol. ii. p 659 



