REPORT ON RADIANT HEAT. 13 
The author then proceeded to try the effect of fine wire gauze 
and fine gratings of cotton thread ; but no difference could be de- 
tected corresponding to the different kinds of heat ; in every case 
the interception 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 tar- 
nished 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 
phenomena in heat, resembling diffraction and periodic co- 
lours in light. 
Such was the general sketch of his researches which Prof. 
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 So- 
ciety, Edinburgh, a more extended account of the entire inves- 
tigations, which appears in vol. xv. Part I. of their Transac- 
tions, 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 re- 
frangible heat, to transparent media, which transmit the red 
rays of light, the transmission of the more refrangible heat being 
analogous to that of violet light. 
Upon this important point Prof. Forbes enlarges in the in- 
troductory part of his memoir; he justly observes that such a 
generalization carries us forward a step, by teaching us to refer 
to the quality of refrangibility certain properties of heat, which 
before were connected 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 dis- 
covery 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 trans- 
mission through glasses of those colours. 
The author gives, as an illustration, the following scale of 
different kinds of heat, in the order of refrangibility, beginning 
with the lowest :— 
