298 
SECOND REPORT— 1832 . 
main difficulty is that of getting any Indication at all, after two 
reflexions from plane surfaces. 
Another point which requires further investigation is the 
apparent transmission of simple heat through very thin trans¬ 
parent screens, but not through opake. This should be ex¬ 
amined in connexion with the acute remark of MM. Nobili 
and Melloni, that a thin stratum of soot may retain its low con¬ 
ducting power, and thus intercept the effect. This of itself 
would form a subject for an accurate series of experiments ; 
viz. whether the ratios of the conducting powers of substances 
remain the same for all thicknesses. 
The very nature of the transmissive and interceptive powers 
of screens is little understood. Supposing simple heat trans¬ 
mitted without diminution, how far is the mode of such trans¬ 
mission analogous to that of light ? wdiat time is required for a 
body to commence radiating heat after it has begun to acquire 
it ? whether it acquires it from a distant source Instantaneously ? 
how the heat distributes itself upon or through a screen ? what 
is precisely the effect of a coating on one side of the screen 
in relation to the last question ? upon what the singular excep¬ 
tions and anomalies pointed out by Melloni and Nobili depend ? 
whether any other such apparently anomalous cases can be 
found?—These are a few of the most obvious questions which 
arise out of the slightest survey of the present state of our know¬ 
ledge, and on which accurate determinations are wanted before 
we can be said to possess even the elements of a scientific 
theory. 
May it not be the law, that if a body be placed in the rays 
from a source of heat, it will be acquiring and giving out heat, 
till the intensity of radiation at the points before and behind it, 
resumes its original proportionality ? 
The time in which this takes place will depend on the extent 
of the body, Its thickness, its conductingpower , its capacity for 
heat , and the state of both Its surfaces. 
These may be such that the effect may be sensibly instanta¬ 
neous, and the radiation therefore appear to go on without in¬ 
terruption. In this case also the distance of the screen from 
the source (within moderate limits,) may make no sensible dif¬ 
ference ; though if any of the above circumstances retard the 
effect to a sensible amount, then there will be a difference with 
the variation of distance. In this way we may as it were regard 
the medium between the source and the thermometer, as merely 
a compound, of which the screen is one portion, and the air 
the other. 
Another class of questions respecting which little if anything 
