274 



INDUCTION 



instances, on the contrary, in which 

 no dew, or but a small quantity of it, 

 is formed, and which are also ex- 

 tremely various, agree (as far as we 

 can observe) in nothing except in not 

 having this same property. We seem, 

 therefore, to have detected the charac- 

 teristic difference between the sub- 

 stances on which dew is produced 

 and those on which it is not produced. 

 And thus have been realised the 

 requisitions of what we have termed 

 the Indirect Method of Difference, or 

 the Joint Method of Agreement and 

 Difference. The example afforded of 

 this indirect method, and of the man- 

 ner in which the data are prepared 

 for it by the Methods of Agreement 

 and of Concomitant Variations, is the 

 most important of all the illustrations 

 of induction afforded by this interest- 

 ing speculation. 



We might now consider the ques- 

 tion, on what the deposition of dew 

 depends, to be completely solved, if 

 we could be quite sure that the sub- 

 stances on which dew is produced 

 differ from those on which it is not, 

 in nothing but in the property of los- 

 ing heat from the surface faster than 

 the loss can be repaired from within. 

 And though we never can have that 

 complete certainty, this is not of so 

 much importance as might at first be 

 supposed ; for we have, at all events, 

 ascertained that even if there be any 

 other quality hitherto unobserved 

 which is present in all the substances 

 which contract dew, and absent in 

 those which do not, this other property 

 must be one which, in all that great 

 number of substances, is present or 

 absent exactly where the property of 

 being a better radiator than conductor 

 is present or absent ; an extent of coin- 

 cidence which affords a strong pre- 

 sumption of a community of cause, 

 and a consequent invariable co-exis- 

 tence between the two properties ; so 

 that the property of being a better 

 radiator than conductor, if not itself 

 the caupe, almost certainly always ac- 

 wmpanies the cause, and, for purposes 

 >{ prediction, no error is likely to 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 in- 

 stance 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 cause of the dew, 

 or its effect. This doubt we are 

 now able to resolve. We have found 

 that, in every such instance, the sub- 

 stance is one which, by its own pro- 

 perties or laws, would, if exposed in 

 the night, become colder than the 

 surrounding air. The coldness there- 

 fore being accounted for indepen- 

 dently of the dew, while it is proved 

 that there is a connection between 

 the two, it must be the dew which 

 depends on the coldness ; or, in other 

 words, the coldness is the cause of 

 the dew. 



This law of causation, already so 

 amply established, admits, however, 

 of efficient additional corroboration 

 in no less than three ways. First, by 

 deduction from the known laws of 

 aqueous vapour when diffused through 

 air or any other gas ; and though we 

 have not yet come to the Deductive 

 Method, we will not omit what is 

 necessary to render this speculation 

 complete. It is known by direct ex- 

 periment that only a limited quantity 

 of water can remain suspended in 

 the state of vapour at each degree 

 of temperature, and that this maxi- 

 mum grows less and less as the tem- 

 perature diminishes. From this it 

 follows, deductively, that if there is 

 already as much vapour suspended as 

 the air will contain at its existing tem- 

 perature, any lowering of that tem- 

 perature will cause a portion of the 

 vapour to be condensed, and become 

 water. But, again, we know deduc- 

 tively, 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 



