EXAMPLES OF THE FOUR METHODS. 497 



than conductor, if not itself the cause, almost certainly 

 always accompanies the cause, and for purposes of 

 prediction, no error will be committed by treating it 

 as if it were really such. 



Reverting now to an earlier stage of the inquiry, 

 let us remember that we had ascertained that, in every 

 instance where dew is formed, there is actual coldness 

 of the surface below the temperature of the surrounding 

 air ; but we were not sure whether this coldness was 

 the cause of dew, or its effect. This doubt we are now 

 able to resolve. We have found that, in every such 

 instance, the substance must be one which, by its own 

 properties or laws, would, if exposed in the night, 

 become colder than the surrounding air. But if the 

 dew were the cause of the coldness, that effect would 

 be produced in other substances, and not solely in 

 those whose own laws suffice to produce it whether 

 there were dew or no. That supposition, therefore, 

 is repelled. But there were only three suppositions 

 possible ; the dew is the cause of the coldness ; both 

 are caused by some third circumstance ; or the cold- 

 ness is the cause of the dew. The first is refuted. The 

 second is inapplicable : the cause of the coldness is a 

 known cause; a radiation from the surface greater 

 than can be supplied by conduction : now this, by its 

 known laws, can produce no direct effect except cold- 

 ness. There remains only the third supposition, that 

 the coldness is the cause of the dew : which, therefore, 

 may be considered as completely made out. 



This law of causation, already so amply established, 

 admits, however, of most efficient additional corrobo- 

 ration in no less than three ways. First, by deduc- 

 tion from the known laws of aqueous vapour when 

 diffused through air or any other gas ; and although 

 we have not yet come to the Deductive Method, we 

 VOL. i. 2 K 



