462 INDUCTION. 



that even if there he 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 coincidence 

 which affords a strong presumption of a community of cause, 

 and a consequent invariable coexistence between the two pro- 

 perties ; so that the property of being a better radiator than 

 conductor, if not itself the cause, almost certainly always 

 accompanies the cause, and, for purposes of 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 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 is one which, 

 by its own properties or laws, would, if exposed in the night, 

 become colder than the surrounding air. The coldness there- 

 fore being accounted for independently of the dew, while it 

 is proved that there is a connexion 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 experiment 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 temperature diminishes. From 

 this it follows, deductively, that if there is already as much 

 vapour suspended as the air will contain at its existing tern- 



