60 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
Let us now proceed to consider the transmission of heat through he- 
terogeneous screens. The calorific rays emerging from each plate ex- 
posed to the action of the same source produce a particular elevation of 
temperature when they fall on the thermoscopic body of our apparatus. 
Whence we have inferred that the guantity of heat which passes through 
a given screen varies according to the quality and thickness of the sub- 
stance. But, it may be asked, is this the only difference between the 
rays immediately transmitted through bodies of different kinds ? 
For the purpose of answering this question we have made the follow- 
ing experiments. 
If the rays from a Locatelli lamp be brought to act on a thermoelec- 
tric pile after having previously passed through a screen of diaphanous 
matter (such as citric acid) but in a slight degree permeable to radiant 
heat, the effect obtained in the ordinary case, in which the whole ac- 
tion is equivalent to 30° of the thermomultiplier, will be very inconsi- 
derable ; but it may be increased by bringing the source of heat nearer, 
or by concentrating its rays on the plate with the help of metallic mir- 
rors or lenses of rock salt. I suppose then that a deviation of 25° or 
30° of the galvanometer has been produced through a plate of citric 
acid. I now interpose a plate of alum in such a manner that the rays 
emerging from the citric acid may be forced to pass through it before 
they can reach the thermoscopic body; the magnetic needle descends 
only about 3 or 4 degrees.” 
I now recommence the operation on any other diaphanous and colour- 
less substance different from the citric acid; that is to say, I vary the 
distance from the lamp to the pile until I obtain the same galvanome- 
tric deviation of 25° or 30° by the action of the radiant heat on this new 
substance also. I then interpose the plate of alum, and the magnetic 
index, as in the case of the citric acid, descends again not more than 
about 3 or 4 degrees, but it approaches nearer to zero, and the retro- 
that caloric and light bear to one another, we have no means of proving that, as 
no polarization of heat is produced by the transmission through the tourmalines, 
none can be produced by reflexion at the surface of the glass. Iam bound also 
to remark that some very able experimental philosophers having lately tried to 
polarize light by M. Bérard’s process, their efforts proved unavailing. Mr. Powell 
informs us that although he had taken the necessary precautions against the 
heating of the glass and other causes of error he has never been able to discover 
the least appearance of polarization when operating with nonluminous heat. 
But he ¢hinks that when he employed luminous sources he was enabled to ob- 
‘serve a small perceptible effect by making the rays previously pass through a 
screen of glass (Edinb. Journal of Science, N. S., vol. vi.) Mr. Lloyd communi- 
cated at the last meeting of the British Association for the Advancement of 
Science (Cambridge 1833) some new results tending to support the conclusions 
derived by Mr. Powell from his own experiments. [No communication upon 
this subject by Professor Lloyd appears in the Report of the British Association 
for 1833.— Ent. | 
