THE EARTH. 313 



an electrical fire, which no watery composition that 

 we know of can effect: these, I say, serve sufficient- 

 ly to excite our wonder ; and still the more, in 

 proportion as the objects are ever pressing on our 

 curiosity. Much, however, has been written con- 

 cerning the manner in which nature operates in 

 these productions j 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 particles of the 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 sus- 

 pended. 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 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.t Dr Hamilton, therefore, of the univer- 

 sity of Dublin, rejecting this theory, has endea- 

 voured to establish another. According to him, 

 as aquafortis is a menstruum that dissolves iron, 

 and keeps it mixed in the fluid ; as aquaregia is 



* Spectacle de la Nature, vol. iii. 



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



