1852.] OF THE ROYAL INSTITUTION. 177 
nosity or combustion ; and that the diversities of heating effect on 
different media, are due to a selective absorption of particular species 
of rays, from peculiarities in the nature of those substances, and 
analogous to the absorption of particular rays of light by coloured 
media. 
It must not however be omitted to notice, however briefly, another 
recent set of researches of high interest, those of M. Silberman; in 
which (among others) the very remarkable fact is established, that 
on transmitting a narrow ray of heat from a heated wire, through 
rock crystal, there is a singular difference according as the ray passes 
parallel or perpendicular to the axis of the crystal: the effect being 
indicated by having the further side of the crystal coated with a fine 
composition of wax, the portion of which in the direction of the ray 
is melted in a circular form in the first instance and in an elliptical 
in the second. 
The general fact of the heterogeneity of heating rays, especially 
from luminous sources, is fully recognized by Melloni as in some 
sense the conclusion from all his experiments. 
The hypothesis that this heterogeneity consists simply in differences 
of wave-length would seem a probable one; though it is still possi- 
ble, as Professor Forbes suggests, that some other element may also 
enter into the conditions. 
This view has been extended by M. Ampére so as to refer both 
luminous and heating effects to the same rays:—a view contro- 
verted by Melloni, chiefly on the ground, evinced by several classes 
of experiments, that the intensity of the heating effect (especially in the 
solar rays) follows no proportion to that of illumination; an argu- 
ment which really amounts to little unless the theory obliged 
us to infer that the amount of illumination must follow the same 
law as that of heat; which it manifestly does not; since the 
nature of the effect in the one case is wholly dependent on the 
unknown constitution of the optic nerve; according to which some 
precise proportion of the impinging vibrations, with a particular 
wave-length, is that which gives the greatest perfection of vision : 
while for heat the effect has no reference to such peculiar conditions, 
but is dependent in some way on longer wave-lengths, and pro- 
bably more simply connected with the intensity or amplitude of the 
vibrations. 
On this theory our view of the case would be thus : — 
A body heated below luminosity begins to give out rays of large 
wave-length only. As it increases in luminosity it continues to send 
out these, and at the same time others of diminishing wave-lengths, 
till at the highest stage of luminosity it gives out rays of all wave- 
lengths from those of the limit greater than the red end of the 
spectrum, to those of the violet end, or possibly less. 
Rays of all these species are transmissible and refrangible by Rock 
salt; and many of them with numerous specific distinctions by other 
media. 
