314 HISTORY OF 



a menstruum that dissolves gold ; or as water dis- 

 solves salts to a certain quantity ; so air is a men- 

 struum that corrodes and dissolves a certain quan- 

 tity of water, and keeps it suspended above. 

 But, however ingenious this may be, it can hard- 

 ly be admitted, as we know, by Mariotte's experi- 

 ment,* that if water and air be enclosed together, 

 instead of the air acting as a menstruum upon 

 the water, the water will act as a menstruum upon 

 the air, and take it all up. We know also, that of 

 two bodies, that which is most fluid and pene- 

 trating is most likely to be the menstruum of the 

 other ; but water is more fluid and penetrating 

 than air, and therefore the most likely of the 

 two to be the menstruum. We know that all 

 bodies are more speedily acted upon, the more 

 their parts are brought into contact with the men- 

 struum that dissolves them, but water, enclosed 

 with compressed air, is not the more diminished 

 thereby.t In short, we know that cold, which 

 diminishes the force of other menstruums, is 

 often found to promote evaporation. In this 

 variety of opinion and uncertainty of conjecture, 

 I cannot avoid thinking that a theory of evapora- 

 tion may be formed upon very simple and obvious 

 principles, and embarrassed, as far as I conceive, 

 with very few objections. 



We know, that a repelling power prevails in na- 

 ture, not less than an attractive one. This repul- 

 sion prevails strongly between the body of fire 

 and that of water. If I plunge the end of a red- 



* Mariotte, de la Nature de I' Air, p. 97. 106. 

 f See Boyle's Works, vol. ii. p. 619. 



