800 INDUCTION. 



air. The coldness, therefore, being accounted for independently 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. 

 /■r " This law of causation, already so amply established, admits, however, of 

 I efficient additional corroboration in no less than three Avays. First, by de- 

 j duction from the known laws of aqueous vapor when diffused through air 

 or any other gas; and though we have not yet come to the Deductive 

 I— Method, we will not omit what is necessary to render this speculation com- 

 plete. It is known by direct experiment that only a limited quantity of 

 water can remain suspended in the state of vapor at each degree of tem- 

 perature, and that this maximum 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 heat, that the contact of the air with a body colder than itself will 

 necessarily lower the temperature of the stratum of air immediately ap- 

 plied to its surface ; and will, therefore, cause it to part with a portion of 

 its water, which accordingly will, by the ordinary laws of gravitation or 

 cohesion, attach itself to the surface of the body, thereby constituting dew. 

 This deductive proof, it will have been seen, has the advantage of at once 

 proving causation as well as co-existence ; and it has the additional advan- 

 tage that it also accounts for the exceptions to the occurrence of the phe- 

 nomenon, 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 under-supplied with aqueous vapoi-, comparatively to its 

 temperature, that even when somewhat cooled by the contact of the cold- 

 er 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 con- 

 dition of the production of dew, which the methods wo previously made 

 use of failed to detect, and which might have remained still undetected, if 

 recourse had not been had to the plan of deducing the effect from the as- 

 certained properties of the agents known to be present. 



The second corroboration of the theory is by direct experiment, accord- 

 ing to the canon of the Method of Difference. We can, by cooling the sui'- 

 face of any body, 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. Here, too, therefore, the causation 

 is directly proved. We can, it is true, accomplish this only on a small 

 scale, but we have ample reason to conclude that the same operation, if 

 conducted in nature's great laboratory, would equally produce the effect. 



And, finally, even on that great scale we are able to verify the result. 

 The case is one of those rare cases, as we have shown them to be, in which 

 nature works the experiment for us in the same manner in which we our- 

 selves perform it ; introducing into the previous state of things a single 

 and perfectly definite new circumstance, and manifesting the effect so rap- 

 idly that there is not time for any other material change in the pre-existing 

 circumstances. " It is observed that dew is never copiously deposited in 

 situations much screened from the open sky, and not at all in a cloudy 

 night; but if the clouds loithdraw even for a feio minutes, and leave a 

 clear opening, a deposition of dew presently begins, and goes on increas- 



