S90 Natural History. 



being satisfactorily developed, yet so much has 

 been done, during the period under consideration, 

 to throw hght upon it, and so many observations 

 and discoveries have been made, either directly or 

 remotely relating to it, that it has, within a few 

 years, assumed an aspect more interesting, prac- 

 tical, and . approaching to the form of a system, 

 than ever before. 



At the commencement of the eighteenth cen- 

 tury, the ascent of water in the atmosphere, in the 

 form of vapour, had been but little investigated, 

 and was very imperfectly understood Nieuentyt 

 and others had taught that the particles of fire, by 

 adhering to those of water, made up violeculce, or 

 small bodies specifically lighter than air. Dr. 

 Halley supposed that by the action of heat, the 

 particles of water are formed into hollow spherules , 

 filled with a finer air, highly rarified, of less spe- 

 cific gravity than the atmosphere, and, of course, 

 disposed to rise in it. While Dr. Desaguliers 

 thought that the ascent of aqueous particles was 

 owing to their being converted into an elastic 

 steam. Such was the state of opinions with regard 

 to this fact, when Dr. Hamilton, of Great-Bri- 

 tain, undertook the investigation of the subject, 

 and proposed a new theory. He held that evapo- 

 ration is the gradual solution of xvater in air, and 

 that the former is suspended in the latter in the 

 same manner as salts, or other soluble substances 

 are suspended in aqueous fluids."' The same doc- 

 trine had been, in substance, suggested before by 

 several philosophers, particularly by M. Le Roy, 

 in 1751; by Dr. Franklin, in 1756; and by 

 Muschenbroeck, in 1769.'' But though these and 



a Essay en the Ascent of Vapours^ &c. This Essay was first read before 

 the Royal Society in 1765, and was afterwards published, with others, under 

 the title of Philosophical Essays, by HuGH HAMILTON, D» D. F. Ri S. 



a Bishop Watson's Cbmital ^itajsj yqI. i. p. 317. 



