252 HISTOhY OF 



than an attractive one. This repulsion prevails strongly he. 

 tvveen 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 in all manner of directions, every part 

 bubbling and steaming until the iron be cold. Why may wc 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 surface ; and as the parts of the 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 

 siu-face be sufficiently increased. This water, thus disengaged 

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

 Dy the reflected rays, and consequently, more adapted for ascend- 

 ing. 



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 steam. 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 measure, assists the quantity of 

 evaporation ; as the quantity of rays are found to be no way 

 diminished thereby. Moist weather alone prevents evapora- 

 tion ; 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 have accounted for the ascent of vapours ; but 

 to account for their falling again, is attended with rather more 

 difficulty. We have already observed, that the particles of va- 

 pour, disengaged from the surface of the water, will be broken 

 and attenuated in their ascent, by the reflected, and even the 



