Proceedings of the British Association. 315 
the others ; and that green glass is the only kind which possesses 
a Béloration for heat, (if we may use the term,) the others acting 
upon it only as more or less transparent glass of uniform tint does 
upon light. From experiments upon the solar rays transmitted 
by green glass, and intercepted by other media, he found they 
passed copiously through rock-salt, but feebly through alum: 
whence he concludes that there are among the solar rays some 
which resemble those of terrestrial heat ; and in general, that the 
differences observed between solar and teviencal heat, as to their 
_ Properties of transmission, are therefore to be attributed merely to 
* the mixture, in different: proportions, of these several species of 
rays. Prof. Forbes repeated and extended Melloni’s experiments 
on the transmission and refraction of heat. One of the most in- 
teresting points to which he directed his attention, was the possi- 
bility ofdetecting heat in the moon’s beams. These, concentra- 
ted by a polyzonal lens of 32°inches diameter, and acting on the 
thermo-multiplier, gave no indication of any effect: so that Prof. 
F. considers it certain that if there be any heat, it must be less 
than the 300,000th part of a degree centigrade. In his third 
section, he investigates the index of refraction for heat of differ- 
ent kinds as compared with that for light in the same medium. 
The method of observation adopted was indirect, depending upon 
the determination of the critical angle of total internal seas 
in a rock-salt prism with two angles of 40°, and one of 1 
an ingenious mechanical contrivance, the sentient eases - the 
pile was made to receive rays coming from the source of heat 
after undergoing two refractions and one reflection, whatever was 
the angle of incidence. The mean of the results obtained from 
Various sources of heat, variously transmitted, for the index of re- 
fraction for rock-salt, is 1.552. The results deduced are:—l1. 
The mean quality, or that of the more abundant proportion of 
the heat from different sources, varies within narrow limits of re- 
frangibility. 2. These limits are very narrow indeed where the 
ditect heat of any source is employed. 3. All interposed media, 
(including those impermeable to light,) so far as tried, raise the 
index of refraction. 4. All the refrangibilities are inferior to that 
of the mean luminous rays. 5. The limits of dispersion are open 
to further inquiry, but the dispersion in the case of sources of 
low temperature, appears to be smaller than that from luminous 
sources 
