A HISTORY OF SCIENCE 



precipitation of watery vapor in any form was made 

 by Dr. W. C. Wells, a man of American birth, whose 

 life, however, after boyhood, was spent in Scotland 

 (where as a young man he enjoyed the friendship of 

 David Hume) and in London. Inspired, no doubt, 

 by the researches of Black, Hutton, and their confreres 

 of that Edinburgh school, Wells made observations on 

 evaporation and precipitation as early as 1784, but 

 other things claimed his attention; and though he as- 

 serts that the subject was often in his mind, he did not 

 take it up again in earnest until about 1812. 



Meantime the observations on heat of Rumford and 

 Davy and Leslie had cleared the way for a proper in- 

 terpretation of the facts about the facts themselves 

 there had long been practical unanimity of opinion. 

 Dr. Black, with his latent-heat observations, had real- 

 ly given the clew to all subsequent discussions of the 

 subject of precipitation of vapor; and from this time on 

 it had been known that heat is taken up when water 

 evaporates, and given out again when it condenses. 

 Dr. Darwin had shown in 1788, in a paper before the 

 Royal Society, that air gives off heat on contracting 

 and takes it up on expanding; and Dalton, in his 

 essay of 1793, had explained this phenomenon as due 

 to the condensation and vaporization of the water con- 

 tained in the air. 



But some curious and puzzling observations which 

 Professor Patrick Wilson, professor of astronomy in 

 the University of Glasgow, had communicated to the 

 Royal Society of Edinburgh in 1784, and some similar 

 ones made by Mr. Six, of Canterbury, a few years later, 

 had remained unexplained. Both these gentlemen ob- 



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