330 M. MELLONI ON THE POLARIZATION OF HEAT, 
lorific radiations will be obtained by the mere inspection of the two ares 
of impulsion which they successively describe on the galvanometer, 
The time required by the needles to arrive at the extremity of these 
ares is from ten to twelve seconds : they do not remain steady until after 
an interval of from eighty to a hundred seconds. Now the sources of 
ployed. That branch of the pile which was surrounded with ice was carefully 
dried and then left exposed to the free action of the air, the other remaining 
still plunged in the water successively raised to different temperatures. In this 
instance the intensities of the electric currents became proportional to the ex- 
cess of the temperature of the water in the vessel over that of the surrounding 
air; for the conducting powers of the bismuth and the antimony in minute 
bars are so very feeble that the heat communicated by the water to one of the 
faces can scarcely reach the other in any such quantity as to excite in it an ap- 
preciable elevation of temperature. 
Although these experiments were repeated with equal success in different at- 
mospheric temperatures, I did not yet consider them perfectly satisfactory. In 
fact, the pile received by contact the differences of temperature which produce 
the electric currents, and in the ordinary mode of using the thermomultiplier 
the differences of temperature arise from the action of radiation. It became 
necessary therefore to demonstrate, by means of radiant heat, that which had 
been proved by means of heat communicated by contact. After the preceding 
experiments, the only question now remaining was this: “ Whether calorific 
rays produce in thermoscopic substances equal dilatations, when they excite in the 
thermomultiplier equal currents of electricity, whatever may be the intensities 
and origin of those rays or the modifications they may have undergone in conse- 
quence of transmission, reflection, or refraction.’ In order to ascertain how far 
this question might be correctly answered in the affirmative, I engaged Mr. 
Bunten to construct an air thermoscope having its reservoirs made of thin cop- 
per, and its dimensions and mounting, as nearly as possible, the same as those 
of the pile of the thermomultiplier. The communications between the reser- 
voirs were established by means of a glass tube, which at first descended very 
obliquely on each side to the horizon, and then took a horizontal direction in 
the intermediate part containing the liquid index: the extreme faces inclosed 
in the metallic appendages were, as well as the interior of these appendages, 
covered with lamp-black: the instrument was lastly furnished with a stem of 
the same thickness as that of the pile, so that the same stand might serve for 
either. I now opened the two tubes of the pile of the thermomultiplier, and 
brought the radiation of a different source to bear on each of its faces; for in- 
stance, that of the Locatelli on the one, and that of copper heated to 400° on 
the other: I then placed the weaker of the two sources more or less near until 
the index of the galvanometer stood fixed at zero. After having thus obtained 
two opposite calorific actions producing electric currents of the same force, I 
removed the pile and put the air thermoscope in its place. The same immobi- 
lity was exhibited by the liquid index. This delicate experiment was repeated 
with the greatest care on different species of radiations; at first on those that 
were direct and rendered more or less intense by a suitable approximation of 
the sources; then on those that were transmitted through plates of different 
kinds or concentrated by means of lenses ; and lastly upon the heat given back 
by reflectors. The result was always the same ; namely, that all calorific radia- 
tions whatsoever, if they excite in the pile equal currents of electricity or equal 
electromagnetic actions, will also produce equal dilatations or temperatures in 
the air thermoscope. (For further details see the report of M. Biot in the x1yth 
volume of Mémoires de ’ Académie des Sciences.) 
