RELATION OF VAPOUR AND AIR. 559 



lently. Huyghens mentions an experiment of the 

 same kind made by Papin about 1673. 



The ascent of vapour was explained in various 

 ways in succession, according to the changes which 

 physical science underwent. It was first distinctly 

 treated of, at a period when hydrostatics had ac- 

 counted for many phenomena ; and attempts were 

 naturally made to reduce this fact to hydrostatical 

 principles. An obvious hypothesis, which brought 

 it under the dominion of these principles, was, to 

 suppose that the water, when converted into vapour, 

 was divided into small hollow globules ; thin pel- 

 licles including air or heat. Halley gave such an 

 explanation of evaporation ; Leibnitz calculated 

 the dimensions of these little bubbles ; Derham 

 managed (as he supposed) to examine them with a 

 magnifying glass; Wolf also examined and calcu- 

 lated on the same subject. It is curious to see so 

 much confidence in so lame a theory ; for if water 

 became hollow globules in order to rise as va- 

 pour, we require, in order to explain the forma- 

 tion of these globules, new laws of nature, which 

 are not even hinted at by the supporters of the 

 doctrine, though they must be far more complex 

 than the hydrostatical law by which a hollow sphere 

 floats. 



Newton's opinion was hardly more satisfactory ; 

 he 3 explained evaporation by the repulsive power 

 of heat; the parts of vapours, according to him, 



3 Opticks, Qu. 31. 



