460 



INDUCTION. 



on 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 cteteris paribus the deposition 

 of dew is in some proportion to the power which the body pos- 

 sesses 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 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 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 appli- 

 cation of the method is, thtt 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 

 promote 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 unfavourable, 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 tex- 

 ture of no substance is absolutely firm or absolutely loose. 

 Looseness of texture, therefore, or something which is the cause 



