EXAMPLES OF THE FOUR METHODS. 



273 



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 phenomenon ; 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 

 obvious. Those polished surfaces 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 

 Concomitant Variations called to our 

 assistance ; and no other method was 

 practicable on this occasion ; for the 

 quality of conducting heat could not 

 be excluded, since all substances con- 

 duct heat in some degree. The 

 conclusion obtained is, that ccttens 

 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, there- 

 fore, (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 in- 

 fluence. 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 requisites for a second employment 

 of the Method of Concomitant Varia- 

 tions, 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 

 cceteris 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, &c., as un- 

 favourable, but those of a loose one, 

 as cloth, velvet, wool, eider-down, 

 cotton, &c., as eminently favourable 

 to the contraction of dew." The 

 Method of Concomitant Variations is 

 here, for the third time, had recourse 

 to, and, as before, from necessity, 

 since the texture of no substance is 

 absolutely firm or absolutely loose. 

 Looseness of texture, therefore, or 

 something which is the cause of that 

 quality, is another circumstance which 

 promotes the deposition of dew ; but 

 this third course resolves itself into 

 the first, viz. the quality of resisting 

 the passage of heat ; for substances 

 of loose texture '* are precisely those 

 which are best adapted for clothing, 

 or for impeding the free passage of 

 heat from the skin into the air, so as 

 to allow their outer surfaces to be 

 very cold, while they remain warm 

 within ; " and this last is, therefore, 

 an induction (from fresh instances) 

 simply corroborative of a former induc- 

 tion. 



It thus appears that the instances 

 in which much dew is deposited, which 

 are very various, agree in this, and, 

 so far as we are able to observe, in this 

 only, that they either radiate heat 

 rapidly or conduct it slowly : qualities 

 between which there is no other cir- 

 cumstance of agreement than that, by 

 virtue of either, the body tends to lose 

 heat from the surface more rapidly than 

 it can be restwed from within. Th« 



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