EXAMPLES OP THE FOUR METHODS. 459 



" We must 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 :" a comparison between instances of dew and in- 

 stances of no dew, being the condition necessary 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 the under side of a 

 horizontal plate of glass is also dewed." Here is an instance 

 in which the effect is produced, and another instance in which 

 it is not produced ; but we cannot yet pronounce, as the 

 canon of the Method of Difference requires, that the latter 

 instance agrees with the former in all its circumstances except 

 one ; for the differences between glass and polished metals are 

 manifold, and the only thing we can as yet be sure of is, that 

 the cause of dew will be found among the circumstances by 

 which the former substance is distinguished from the latter. 

 But if we could be sure that glass, and the various other sub- 

 stances on which dew is deposited, have only one quality in 

 common, and that polished metals and the other substances 

 on which dew is not deposited have also nothing in common 

 but the one circumstance, of not having the one quality which 

 the others have ; the requisitions of the Method of Difference 

 would be completely satisfied, and we should recognise, in that 

 quality of the substances, the cause of dew. This, accordingly, 

 is the path of inquiry which is next to be pursued. 



"In the cases of polished metal and polished glass, the 

 contrast shows evidently that the substance has much to do 

 with the phenomenon ; therefore let the substance alone be 

 diversified as much as possible, by exposing polished surfaces 

 of various kinds. This done, a scale of intensity becomes 

 obvious. Those polished substances are found to be most 

 strongly dewed which conduct heat worst ; while those which 

 conduct well, resist dew most effectually." The complication 

 increases ; here is the Method of Concomitant Variations 

 called to our assistance ; and no other method was practicable 



