246 INDUCTION. 



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 siairounding 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 

 coldness 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 coldness. 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, how- 

 ever, of most efficient additional corroboration in no less than three 

 vvays. First, by deduction from the known laws of aqueous vapor 

 when diffused through air or any other gas ; and although we have 

 not yet come to the Deductive Method, we will not omit what is neces- 

 sary to render this speculation complete. It is known by direct exper- 

 iment that only a limited quantity of water can remain suspended in 

 the state of vapor at each degi-ee 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 vapor suspended 

 as the air will contain at its existing temperature, any lowering of that 

 temperature will cause a portion of the vapor to be condensed and 

 become water. But, again, we know deductively, from the laws of 

 beat, 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 with a portion 

 of its water, which accordingly will, by the ordinary laws of gravita- 

 tion or cohesion, attach itself to the surface of the body, thereby con- 

 stituting dew. This deductive proof, it will have been seen, has the 

 advantage of proving at once, causation as well as coexistence;' and it 

 has the additional advantage that it also accounts for the cxcejytions to 

 the occurrence of the phenomenon, the cases in which, although the 

 body is colder than the air, yet no dew is deposited; by showing that 

 this will necessarily be the case when the air is so imdersupplied with 

 aqueous vapor, comparatively to its temperature, that even when some- 

 what cooled by the contact of the colder body, it can still continue to 

 hold in suspension all the vapor which was previously suspended in it: 

 thus in a very dry summer there are no dews, in a very dry winter no 

 hoar frost. Here, therefore, is an additional condition of the produc- 

 tion of dew, which the methods we previously made use of failed to 

 detect, and which might have remained still undetec*ted, if recourse had 

 not been had to the plan of deducing the effect from the ascertained 

 properties of the agents known to be present. 



The second corroboration of the theory is by direct experiment, 

 accoi-ding to the canon of the Method of Difference. We can, by 

 cooling the surface of anybody, find in all cases some temperature 

 (more or less inferior to that of the surrounding air, according to its 

 hygrometric condition,) at which dew will begin to be ^deposited. 



