EXAMPLES OF THE FOUR METHODS. 463 



perature, 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 

 gravitation 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 at once proving causa 

 tion as well as coexistence ; and it has the additional advan 

 tage 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- sup 

 plied 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 previously 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 production 

 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 had to the plan of deducing the effect 

 from the ascertained properties of the agents known to be 

 present. 



The second corroboration of the theory is by direct experi 

 ment, according to the canon of the Method of Difference. We 

 can, by cooling the surface of any body, find in all cases some 

 temperature, (more or less inferior to that of the surrounding 

 air, according to its hygrometric condition,) at which dew will 

 begin to be deposited. Here, too, therefore, the causation is 

 directly proved. We can, it is true, accomplish this only on 

 a small scale ; but we have ample reason to conclude that the 

 same operation, if conducted in Nature s great laboratory, 

 would equally produce the effect 



And, finally, even on that great scale we are able to verify 

 the result. The case is one of those rare cases, as we have 



