492 INDUCTION. 



exist in this case ? " Is it a fact that the object 

 dewed is colder than the air? Certainly not, one 

 would at first be inclined to say ; for what is to make 

 it so ? But .... the experiment is easy : we have only 

 to lay a thermometer in contact with the dewed sub- 

 stance, and hang one at a little distance above it, out 

 of reach of its influence. The experiment has been 

 therefore made ; the question has been asked, and 

 the answer has been invariably in the affirmative. 

 Whenever an object contracts dew, it is colder than 

 the air." 



Here then is a complete application of the 

 Method of Agreement, establishing the fact of an inva- 

 riable connexion between the deposition of dew on a 

 surface, and the coldness of that surface compared 

 with the external air. But which of these is cause 

 and which effect ? or are they both effects of some- 

 thing else ? On this subject the Method of Agreement 

 can afford us no light : we must call in a more potent 

 method. 



"That dews are accompanied with a chill is a 

 common remark ; but vulgar prejudice would make 

 the cold the effect rather than the cause. We must 

 therefore collect more facts, or, which comes to the 

 same thing, vary the circumstances ; since every 

 instance in which the circumstances differ is a fresh 

 fact ; and especially, we must note the contrary or 

 negative cases, i. e., where no dew is produced :" for 

 we are aware that a comparison between instances of 

 dew and instances of no dew, is the condition neces- 

 sary to bring the Method of Difference into play. 



" Now, first, no dew is produced on the surface of 

 polished metals, but it is very copiously on glass, both 

 exposed with their faces upwards, and in some cases 



