272 



INDUCTION. 



precisely what we mean by dew ; what 

 the fact really is, whose cause we 

 desired to investigate, "We must 

 separate dew from rain, and the 

 moisture of fogs, and limit the appli- 

 cation of the term to what is really 

 meant, which is the spontaneous 

 appearance of moisture on substances 

 exposed in the open air when no 

 rain or visible wet is falling." This 

 answers to a preliminary operation 

 which will be characterised in the 

 ensuing book, treating of operations 

 subsidiary to induction.* 



" Now, here we have analogous 

 phenomena in the moisture which 

 bede\vs a cold metal or stone when 

 we breathe upon it ; that which ap- 

 pears on a glass of water fresh from 

 the well in hot weather ; that which 

 appears on the inside of windows 

 when sudden rain or hail chills the 

 external air ; that which runs down 

 our walls when, after a long frost, a 

 warm moist thaw comes on." Com- 

 paring these cases, we find that they 

 all contain the phenomenon which 

 was proposed as the subject of in- 

 vestigation. Now '* all these instances 

 agree in one point, the coldness of the 

 object dewed, in comparison with the 

 air in contact with it." But there 

 still remains the most important case 

 of all, that of nocturnal dew : does 

 the same circumstance exist in this 

 case ? " Is it a fact that the object 

 dewed is colder than the air? Cer- 

 tainly not, one would at first be in- 

 clined to say ; for what is to maJce it 

 so? But .... the experiment is 

 easy : we have only to lay a thermo- 

 meter in contact with the dewed 

 substance, 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 in- 

 variably in the affirmative. When- 

 ever an object contracts dew, it is 

 colder than the air." 



Here then is a complete application 

 of the Method of Agreement, esta- 



• Infra, book iv. cb. ii. On Abstraction. 



blishing the fact of an invariable con- 

 nection 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 something else? On this subject 

 the Method of Agreement can afford 

 us no light : we must call in a more 

 potent method. "We must collect 

 more facts, or, which comes to the 

 same thing, vary the circumstances ; 

 since every instance in which the cir- 

 cumstances 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 instances 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 ex- 

 posed 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 be- 

 tween 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 circum- 

 stances by which the former substance 

 is distinguished from the latter. But 

 if we could be sure that glass, and the 

 various other substances 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 sub- 

 stances, the cause of dew. This, 



