494 INDUCTION. 



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 well,, resist dew most effectually." The 

 complication increases ; here is the Method of Conco- 

 mitant Variations called to our assistance ; and no 

 other method 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 c<eteris paribus 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 con- 

 nected with this,) must be at least one of the causes 

 which assist in producing the deposition of dew upon 

 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 Difference to ascertain concomitance of 

 variations,) " and another scale of intensity becomes 

 at once apparent; those surfaces which part with their 

 heat most readily by radiation, are found to contract 

 dew most copiously." Here, therefore, are the requi- 

 sites for a second employment of the Method of Con- 

 comitant 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 c(teris paribus the deposition of dew is also in 

 some proportion to the power of radiating heat ; and 



