RADIANT HEAT. 343 



from that which these media transmit more readily; that is, as the 

 temperature of the source is higher. The following results were 

 stated: 



Heat from lamp through Rays out of 100 



smoked rock salt. transmitted. 



Direct 36 



Previously sifted 4>y another plate of smoked rock salt- 44 



Do do laminated mica • • 44 



Do do roughened salt • • 40.\ 



The author then proceeded to try the effect of fine wire gauze and 

 fine gratings of cotton thread, but no difference could be detected cor- 

 esponding to the different kinds of heat; in every case the intercep- 

 tion was proportioned to the fineness of the gauze. 



When fine powders were strewed between plates of rock salt, or fine 

 lines were ruled upon the surface, or the surface tarnished by mere 

 exposure to the air, the easier transmission of heat of low temperature 

 was rendered apparent. 



These effects the author considers as evidently pointing to phenom- 

 ena in heat resembling diffraction and periodic colors in light. 



Such was the general sketch of his researches which Professor 

 Forbes gave at the period above mentioned. Subsequently (up to 

 March, 1840) he continued engaged on the same subjects, and on 

 May 15, 1840, laid before the council of the Royal Society, Edinburgh, 

 a more extended account of the entire investigations, which appears 

 in vol. xv, Part I, of their Transactions, under the title of " A Fourth 

 Series of Researches on Heat." Some remarks by M. Melloni appear 

 in the Comptes Rendus, March 30, 1840, on the same subject. 



For obtaining a general view of these results the main point to be 

 kept in sight is the relation which the transmissibility of each sort of 

 heat appears to bear to its refrangibility; and hence the analogy of 

 diathermanous media, which transmit the less refrangible heat, to 

 transparent' media, which transmit the red rays of light, the trans- 

 mission of the more refrangible heat being analogous to that of violet 

 light. 



Upon this important point Professor Forbes enlarges in the intro- 

 ductory part of his memoir; he justly observes that such a generaliza- 

 tion carries us forward a step, by teaching us to refer to the quality 

 of refrangibility certain properties of heat which before were con- 

 nected only with certain vague characters in the nature of the source 

 whence it was derived. Among other things we find, what was long 

 suspected, but what Melloni first conclusively proved, that it does not 

 essentially depend on the presence or absence of light. This refers 

 to his singular discovery of the change produced by the intervention 

 of certain screens. 



Heat from any source, if it admit of transmission at all through 

 glass, alum, or water, will ultimately have the character of glass-heat, 

 alum-heat, or water-heat, just as light from the sun or from a candle 

 becomes red, blue, or green, by transmission through glasses of those 

 colors. 



