THE STORY OF NINETEENTH-CENTURY SCIENCE 



of evaporation, yet he still clings to the idea that the 

 vapor, though independent of the air, is combined with 

 particles of caloric. He holds that clouds are composed 

 of vapor that has previously risen from the earth, com- 

 bating the opinions of those who believe that they are 

 formed by the union of hydrogen and oxygen existing 

 independently in the air; though he agrees with these 

 theorists that electricity has entered largely into the 

 modus opemndi of cloud formation. He opposes the 

 opinion of Deluc and de Saussure that clouds are com- 

 posed of particles of water in the form of hollow vesicles 

 (miniature balloons, in short, perhaps filled with hydro- 

 gen), which untenable opinion was a revival of the theory 

 as to the formation of all vapor which Dr. Halley had 

 advocated early in the eighteenth century. 



Of particular interest are Howard's views as to the 

 formation of dew, which he explains as caused by the 

 particles of caloric forsaking the vapor to enter the cool 

 body, leaving the water on the surface. This comes as 

 near the truth perhaps as could be expected while the 

 old idea as to the materiality of heat held sway. How- 

 ard believed, however, that dew is usually formed in 

 the air at some height, and that it settles to the surface, 

 opposing the opinion, which had gained vogue in France 

 and in America (where Noah Webster prominently ad- 

 vocated it), that dew ascends from the earth. 



The complete solution of the problem of dew forma- 

 tion which really involved also the entire question of 

 precipitation of watery vapor in any form was made 

 by Dr. C. W. 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 re- 



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