298 INDUCTION. 



it is very copiously on glass, both exposed with their faces upward, 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 can not 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 between 

 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 circumstances 

 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 recognize, in that quality of the sub- 

 stances, the cause of dew. This, accordingly, is the path of inquiiy which 

 is next to be pursued. 



" In the cases of polished metal and polished glass, the contrast shows 

 evidently that the substance has much to do with the phenomenon ; there- 

 fore let the substance alone be diversified as much as possible, by exposing 

 polished surfaces of various kinds. This done, a scale of intensity becomes 

 obvious. Those polished substances are found to be most strongly dewed 

 which conduct heat worst; while those which conduct heat well, resist dew 

 most effectually." The complication increases ; here is the Method of 

 Concomitant Variations called to our assistance ; and no other metliod was 

 practicable on this occasion ; for the quality of conducting heat could not 

 be excluded, since all substances conduct heat in some degree. The conclu- 

 sion obtained is, that cmteris paribus the deposition of dew is in some pro- 

 portion to the power which the body possesses of resisting the passage of 

 heat; and that this, therefore (or something connected with this), must be 

 at least one of the causes which assist in producing the deposition of dew 

 on the surface. 



"But if we expose rough surfaces instead of polished, we sometimes find 

 this law interfered with. Thus, roughened iron, especially if painted over 

 or blackened, becomes dewed sooner than varnished paper; the kind of 

 surface, therefore, has a great influence. Expose, then, the same material 

 in very diversified states, as to surface " (that is, employ the Method of Dif- 

 ference to ascertain concomitance of variations), " and another scale of in- 

 tensity becomes at once apparent ; those surfaces which part icith their 

 heat most readily by radiation are found to contract dew most copiously." 

 Here, therefore, are the requisites for a second employment of the Method 

 of Concomitant Variations ; which in this case also is the only method 

 available, since all substances radiate heat in some degree or other. The 

 conclusion obtained by this new application of the method is, that coeteris 

 paribus the deposition of dew is also in some proportion to the power of 

 radiating heat; and that the quality of doing this abundantly (or some 

 cause on which that quality depends) is another of the causes which pro- 

 mote the deposition of dew on the substance. 



"Again, the influence ascertained to exist of substance and surface leads 

 us to consider that of texture : and here, again, we are presented on trial 

 with remarkable differences, and with a third scale of intensity, pointing 

 out substances of a close, firm texture, such as stones, metals, etc., as un- 

 favoi-able, but those of a loose one, as cloth, velvet, wool, eider-down, cot- 



