RELATION OF VAPOR AND AIR. 105 



Sect. -. Prelude to D< tiffin's Doctrine of Evf.poration. 



V T ISIBLE clouds, smoke, distillation, gave the notion of Vapor ; vapor 

 was at first conceived to be identical with air, as by Bacon. 3 It was 

 easily collected, that by heat, water might be converted into vapor. 

 It was thought that air was thus produced, in the instrument called 

 the oeolipile, in which a powerful blast is caused by a boiling fluid ; but 

 "Wolfe showed that the fluid was not converted into air, by using cam- 

 phorated spirit of wine, and condensing the vapor after it had been 

 formed. We need not enumerate the doctrines (if very vague hypo- 

 theses may be so termed) of Descartes, Dechales, Borelli. 4 The latter 

 accounted for the rising of vapor by supposing it a mixture of fire and 

 water ; and thus, fire being much lighter than air, the mixture also 

 was light. Boyle endeavored to show that vapors do not permanently 

 float in vacua. lie compared the mixture of vapor with air to that 

 of salt with water. He found that the pressure of the atmosphere- 

 affected the heat of boiling water; a very important fact. Boyle 

 proved this by means of the air-pump ; and he and his friends were 

 much surprised to find that when air was removed, water only just 

 warm boiled violently. Huyghens mentions an experiment of the 

 same kind made by Papin about 1673. 



The ascent of vapor was explained in various ways in succession, 

 according to the changes which physical science underwent. It was a 

 problem distinctly treated of, at a period when hydrostatics had 

 accounted 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 sup- 

 pose that the water, when converted into vapor, was divided into 

 small hollow globules ; thin pellicles including air or heat. Halley 

 U'ave 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: Wolfe also examined and 

 calculated on the same subject. It is curious to see so much confi- 

 dence in so lame a theory; for if water became hollow globules in 

 order to rise as vapor, we require, in order to explain the formation of 

 these globules, new laws of nature, which are not even hinted at In 



3 Bacon's Hist. Nat. Cent. i. p. 27. 



4 They may be seen in Fischer, Gcschichte dcr Phys'tk, vol. ii. p. 175. 



