30 REPORT—1840. 
In an attempt to pursue the analogy of the tints of depolar- 
ized light, he acknowledges a failure, and thence considers the 
interference of calorific rays as not yet proved. 
Upon these investigations the following remarks may be 
offered :— 
Under the first head, it should be recollected that Prof. 
Forbes’s first memoir, was avowedly only directed to ascertain 
general facts, not numerical values; while, with regard to the 
more precise results of the second memoir, it would appear from 
the details there given, that the secondary radiation could not 
affect the results. The screen between the source and the piles 
was removed only during the few seconds required for observing 
the first or impulsive arc of vibration, the time of which was 
wholly insufficient for the conduction of heat; besides, such an 
effect was disproved by direct experiment, as mentioned above 
(p. 23). 
3 Of the second point, we shall presently have to notice a com- 
plete investigation by Prof. Forbes. 
As to the third, with this construction, the heat absorbed by 
the mica was very trifling; but by the more improved process 
since used by Prof. Forbes, (p. 27), we have seen this source of 
error is wholly got rid of. The employment of a pencil of 
parallel rays does not seem, upon consideration, materially to 
increase the intensity. 
The fourth point is no more than what had been already 
established by Prof. Forbes. 
With respect to the fifth head (including the most important 
part of these researches), it must be observed, that the differ- 
ences in the nature of the heat obtained by the intervention of 
diathermanous substances, are not the same as those between heat 
from luminous and dark sources. And further, in the experi- 
ments mentioned with radiations from different sources, no 
numerical results are stated. On this point we shall presently 
notice some more detailed researches of Prof. Forbes. 
The sixth point, on the subject of depolarization, is confess- 
edly one of the most delicate in the whole inquiry; but for the 
same reasons as before, the effect of secondary radiation cannot 
be referred to as capable of having produced the differences 
observed. 
Unequal Polarizability of Heat from different sources : 
Forbes. 
Prof. Forbes’s third series of Researches of Heat appears in 
vol. xiv. of the Edinburgh Transactions, having been read before 
the Royal Society of Edinburgh, April 16, 1838. It is also 
