340 
REPORT —1854. 
A strong and curious confirmation of the heating power derived from 
luminiferous rays simply, has been furnished in a tact mentioned to me bv 
Dr. Bennett, Professor of Physiology in the University of Edinburgh; viz. 
that in the exhibition of anatomical subjects by means of the oxy-hydrogen mi¬ 
croscope, the light concentrated by a lens, at a few inches distance from the 
source, had so energetic an action as to burn up and destroy the specimens 
placed in the focus. 
On the Theoretical Explanation of some former Experiments. 
The experiments of Mellont (of which some general abstracts are given in 
my former Report (1840, p. 4)), certainly seem at first sight to present some 
anomalous results. 
The main question which seems to arise is, whether the effects of heat, as 
compared with those of illumination, do not follow such widely discrepan 
laws, as to make it difficult to ascribe them to the same set of wares; an 
this both with regard to terrestrial and to the solar heat. 
To take a single instance: rack salt is said to be analogous, for uem, to 
colourless media for light ; alum is described as totally impermeable to 
“dark heat,” and partially so to the rays from a lamp, that is,*tina? 
wholly impermeable to those rays of the lamp which are identical with • ir ‘ 
heat" (species J.l in their relation to absorption, according to the texture » 
surfaces, and wholly permeable to those (Species II.) w hich are nssocia 
with light , and produce this effect in proportion to the absorption of hgut )’ 
dark coloured surfaces; or in the language of the wave-theory, wholly im¬ 
permeable to rays of longer wave-length, and wholly permeable to cava 
rays of smaller wave-length. , , ... 
In some of Melloni’s experiments, rays from the lamp transmitted 
ferentproportions by various screens, and then equalized, were afferwar s 
found to be transmitted by alum in similar proportions. This lie uescri 
by the expression that “ they possess the diathermancy peculiar to the to- 
stances through which they had passed." Yet the fact surely implies none 
property communicated to the rays. It merely shows, that as different^ 1 ^ 
rays out of the compound beam were transmitted in each case by the 
screen, alum, though impervious to the lower heating rays (i. e. of lower 
frangibility or longer wave-length), is permeable to those higher rays;® 1 
diff erent degrees according to their nature-, an effect simply dependent on 
heterogeneity of the compound beam from the flame. Again, wi i 
ferently coloured glasses peculiar differences of diathermancy were exJ'i 1 
with the rays from a lamp, incandescent metal, and the sun; but no 01 
various or anomalous than the absorption of specific rays of light by sue 
media. And besides these considerations, it must be borne in mind t 
smooth blackened surface is itself unequally absorptive for th ? different W 
acting (from its colour) more energetically on those of a refrangibilitv wi 
the limits of the visible spectrum, and which affect the eye as rays of Ug - 
and more feebly on the rays of lower relrangibility, and which act more en 
getically on bodies with reference to the absorptive ktehto of their «rla** 
As (according to my experiments) the solar heat is wholly of tha* 
which is freely transmitted through all colonibss media along with the l«g 
it does not appear that there would be any particular advantage in opera * 
on the solar spectrum with a rock-salt prism. Mellon!, however, 
a prism found, on interposing a thick screen of water, the most beahng • 
(*.c. those towards the red end) intercepted (as they are known to be 
water), and tins caused the position of the maximum to be apparently 
uglier up the spectrum, even to the position of the green rays. . 
On the other hand, many coloured glasses, lie found, absorbed the ra)- 
