498 INDUCTION. 



will not omit what is necessary to render this specula- 

 tion complete. It is known by direct experiment that 

 only a limited quantity of water can remain suspended in 

 the state of vapour at each degree of temperature, and 

 that this maximum grows less and less as the tempe- 

 rature diminishes. From this it follows, deductively, 

 that if there is already as much vapour suspended as 

 the air will contain at its existing temperature, any 

 lowering of that temperature will cause a portion of 

 the vapour to be condensed, and become water. But, 

 again, we know deductively, from the laws of heat, 

 that the contact of the air with a body colder than 

 itself, will necessarily lower the temperature of the 

 stratum of air immediately applied to its surface ; and 

 will therefore cause it to part with a portion of its water, 

 which accordingly will, by the ordinary laws of gravita- 

 tion or cohesion, attach itself to the surface of the body, 

 thereby constituting dew. This deductive proof, it 

 will have been seen, has the advantage of proving at 

 once, causation as well as coexistence ; and it has the 

 additional advantage that it also accounts for the 

 exceptions to the occurrence of the phenomenon, the 

 cases in which, although the body is colder than the 

 air, yet no dew is deposited; by showing that this 

 will necessarily be the case when the air is so under- 

 supplied with aqueous vapour, comparatively to its 

 temperature, that even when somewhat cooled by the 

 contact of the colder body, it can still continue to 

 hold in suspension all the vapour which was pre- 

 viously suspended in it : thus in a very dry summer 

 there are no dews, in a very dry winter no hoar frost. 

 Here, therefore, is an additional condition of the pro- 

 duction of dew, which the methods we previously 

 made use of failed to detect,, and which might have 

 remained still undetected, if recourse had not been 



