100 



A HISTORY OF 



however, has been written concerning the 

 manner in which nature operates in these 

 productions ; as nothing is so ungrateful to 

 mankind as hopeless ignorance. 



And first, with regard to the manner in 

 which water evaporates, and rises to form 

 clouds, much has been advanced, and many 

 theories devised. All water," say some, has 

 a quantity of air mixed with it ; and the heat 

 of the sun darting down, disengages the par- 

 ticles of this air from the grosser fluid ; 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 fire, 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. b Dr. Hamilton, therefore, of the uni- 

 versity of Dublin, rejecting this theory, has 

 endeavoured to establish another. According 

 to him, as aqua-fortis is a menstruum that dis- 

 solves 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 quantity of water, and 

 keeps it suspended above. But however in- 

 genious this may be, it can hardly be admit- 

 ted : as we know by Mariotte's experiment," 

 that if water and air be inclosed together, in- 

 stead 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 

 parts are brought into contact with the men- 

 struum that dissolves them ; but water, in- 



a Spectacle de la Nature, vol. iii. 



b Memoires de 1' Academic des Sciences, an. 1705. 



closed with compressed air, is not the mdre 

 diminished thereby.* 1 In short, we know that 

 cold, which diminishes the force of other men- 

 struuras, is often found to promote evapora- 

 tion. In this variety of opinion and uncer- 

 tainty 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 in all manner of directions, every part 

 bubbling and steaming until the iron be cold. 

 Why may we not ibr a moment compare the 

 rays of the sun, darted directly upon the sur- 

 face of the water, to so many bars of red-hot 

 iron, each bar indeed infinitely small, but 

 not the less powerful ? In this case, wher- 

 ever a ray of fire darts, the water, from its re- 

 pulsive 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 extreau-ly small ; 

 as the ray that darts is extremely so. The 

 assemblage of the rays darting upon the water 

 is this manner, will cause it to rise in a light 

 thin steam above the surface ; 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 in- 

 creased. This water, thus disengaged from 

 the general mass, will be still farther atten- 

 uated and broken by the reflected rays, and 

 consequently more adapted for ascending. 



From this plain account, every appearance 

 in evaporation may be easily deduced. The 

 quantity of heat increases evaporation, be- 

 cause it raises a greater quantity of steam. 

 The quantity of wind increases evaporation ; 

 for, by waving the surface of the wateryjt thus 



c Mariotte, de la Nature de 1'Air, p. 97, 106. 

 * See Boyle'* Works, vol. ii. p. 6l9. 



