REPORT ON RADIANT HEAT. 5 
the same thickness; most of them were wholly impervious to 
dark heat ; the most remarkable exceptions being fluate of lime 
and rock salt. 
In another table are the results with black glass and black 
mica; these substances, though diathermanous to the lamp and 
incandescent platina, are wholly impervious to the rays from 
hot water, and nearly so to those from heated copper. 
The discovery of.the entire diathermancy of rock salt has 
been before referred to, and has furnished the means of prose- 
cuting the author’s yet more remarkable researches on the RE- 
FRACTION OF Hear. 
Tothisimportant point M. Mellonidevotes a pertion ofthe same 
memoir. After a sketch of previous attempts to establish this 
property, he describes his successful experiment by concentra- 
ting to the focus of a rock-salt lens the rays of dark heat from 
hot copper and hot water. 4 similar lens of alum produced 
no effect; this proves that the effect is not due to the mere heat- 
ing of the central part of the lens. 
He next advances to the refraction of heat by a rock-salt 
prism ; describing an apparatus for the purpose. That the ef- 
fect is not due to secondary radiation, is shown by turning the 
prism on its axis into a different position, when no effect is 
roduced. 
He then discusses the “ properties of the calorific rays imme- 
diately transmitted by different bodies.’’ Under this head are 
detailed one of the most remarkable species of effects which the 
whole range of the subject presents. 
_The rays of the lamp were thrown upon screens of different 
substances in such a manner, that either by changing the di- 
stance, or by concentration with a mirror, or a lens of rock salt, 
the effect transmitted from all the screens was of a certain con- 
stant amount. This constant radiation was then intercepted 
by a plate of alum, and it was found that very different propor- 
tions of heat were transmitted by the alum in the different cases. 
This very singular result is established by numerous detailed 
experiments, of which a tabular statement is given, and the au- 
thor states it in the following terms: “‘the calorific rays issuing 
from the diaphanous screens are therefore of different qualities, 
and possess, if we may use the term, the diathermancy peculiar 
to each of the substances through which they have passed.” 
_ He next investigates the effects of different colours in glass 
on the absorption of heat. He infers in general that the colour- 
ing matter diminishes the power of transmission, and examines. 
the question, does it stop only rays of a definite refrangibility 
analogous to what happens in the absorption of light ? 
