24 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



54. The leading facts contained in this paper are these : 



55. I. The peculiar (red-like) character of films of smoke in transmitting heat 

 of low temperature is partaken, 



A. By simple powder of charcoal. 



B. By (at least some) other dull earthy powders. 



C. By surfaces simply dull or devoid of polish. 



D. By surfaces irregularly furrowed, as with emery or sand-paper. 



E. By polished surfaces, on which fine distinct lines have been drawn. 



F. By the mechanical lamination of transparent mica, which, as a conti- 



nuous medium, possesses opposite properties. 



56. II. The following media seem indifferent to the kind of heat which they 

 transmit : 



A. The thinnest gold-leaf is impervious to any. 



B. Metallic gratings transmit all kinds of heat in a proportion which is pro- 



bably exactly as the area of the interstices which they present. 



C. Thread gratings. 



D. In a state of powder, most crystalline bodies approach to a condition of 



opacity for heat. 



57. III. The following bodies, in addition to those commonly kno'wn, trans- 

 mit most heat of high temperature (violet-like heat). 



A. Several pure metallic powders. 



B. Rock-salt in powder ; and many other powders. 



C. Animal membrane. 



58. IV. Heat of low temperature is most regularly reflected at imperfectly 

 polished surfaces. It is also, we have seen, most regularly transmitted. These 

 facts are of great importance to the Theory of Heat ; and may probably suggest 

 inquiries of no small interest with regard to light, and especially the pheno- 

 mena of absorption. 



59. We have already (24) noticed the analogy which the fact stated in the 

 last article bears to the easier reflection of red than violet light from depolished 

 surfaces, and in that fact we find a confirmation of the application of the undu- 

 latory doctrine to heat, and of the opinion that the waves producing heat, are 

 longer in proportion as the temperature of the source is less. The phenomena of 

 transmission are more obscure ; they may be compared either to the Diffraction, 

 or to the Absorption, of light. 



60. The action of lines on polished surfaces, similar to those used in many 

 diffraction experiments, led to the inquiry (31), whether the mean colour of light 

 transmitted by gratings was necessarily unchanged ? The question does not seem 

 to have occurred to any one to whom I have mentioned it ; and though the most 

 likely result would seem to be, that there should be no change, the grounds of such 

 an a priori opinion do not appear absolutely conclusive. Professor KELLAND, how- 



