OF NATURAL PHILOSOPHY. 161 



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 (Rule 5. 152.). 

 Those polished substances are found to be most 

 strongly dewed which conduct heat worst; while 

 those which conduct well resist dew most effectually. 

 Here we encounter a law of the first degree of gene- 

 rality. But, if we expose rough surfaces, instead of 

 polished, we sometimes find this law interfered with 

 (Rule 5. 152.). 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 ma- 

 terial in very diversified states as to surface (Rule 7- 

 156.), and another scale of intensity becomes at once 

 apparent ; those surfaces which part with their Jteat 

 most readily by radiation are found to contract 

 dew most copiously : and thus we have detected 

 another law of the same generality with the former, 

 by a comparison of two classes of facts, one relating 

 to dew, the other to the radiation of heat from 

 surfaces. 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 unfavour- 

 able, but those of a loose one, as cloth, wool, velvet, 

 eiderdown, cotton, &c. as eminently favourable, to 

 the contraction of dew : and these are precisely those 

 which are best adapted for clothing, or for impeding: 



