244 INDUCTION. 



duced, and another instance in which it is not produced ; but we cannot 

 yet pronounce, as the canon of the Method of Difference requires, 

 that the latter instance agrees with the former in all its circumstancea 

 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 substances, the cause of dew. This, accordingly, 

 is the path of inquiry 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 phenome- 

 non ; therefore let the substance alone be diversified as much as 

 possible, by exposing polished surfaces of various kinds. This done, 

 a scale of intensity becomes obNaous. Those polished substances are 

 found to be most strongly dewed which conduct heat worst ; while 

 those which conduct well, resist dew most effectually." The compli- 

 cation increases ; here is the Method of Concomitant Variations called 

 to our assistance ; and no other metliod was practicable upon this 

 occasion ; for the quality of conducting heat could not be excluded, 

 since all substances conduct heat in some degree. The conclusion 

 obtained is, that cceteris jjaribus the deposition of dew is in some 

 proportion 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 ef the causes which assist in producing the 

 deposition of dew upon the surface. 



" But if we expose rough surfaces instead of polished, we some- 

 times find this law interfered with. Thus, roughened iron, especially 

 if painted over or blackened, becomes dewed sooner tlian varaished 

 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 Difference to ascertain concomitance of 

 variations,) "and another scale of intensity becomes at once apparent; 

 those stirfaccs which jiart with 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 ca.teris 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 

 promote the deposition of dew upon 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 finn textui-e, such as stones, metals, 

 &c., as unfavorable, but those of a loose one, as cloth, wool, velvet, eider- 



