PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
367 
The reasoning just employed leads to the inference that those molecules which expe- 
rience most resistance from the ether, must be the least competent to transfer the motion 
of heat from one to the other. The direct power of communication is enfeebled by the 
ether, and the motion obtained indirectly cannot make good the loss. We are thus led 
to the conclusion that the best radiators ought to prove themselves the worst con- 
ductors. 
A broad consideration of the subject shows that the conclusion is in general harmony 
with observed facts. Organic substances are all exceedingly imperfect conductors of 
heat, and they are all excellent radiators. The moment we pass from the metals to 
their compounds we pass from a series of good conductors to bad ones, and from a series 
of bad radiators to good ones *. 
In the earlier memoirs of MM. De le Provostaye and Desains^, and in that of MM. 
Wiedemann and Franz, I find the following facts : — The radiative power of platinum is 
five times that of silver ; its conductive power is one-tenth that of silver. Platinum has 
more than twice the radiative power of gold; it has only one-seventh of the conducting- 
power. Zinc and tin are almost equal as conductors, and they are also nearly equal as 
radiators. Silver has about six times the conductive power of zinc and tin ; it has only 
one-fourth of their radiative powers. Brass possesses but one-half the radiative energy 
of platinum ; it possesses more than twice its conductivity. Other experiments of 
MM. De la Provostaye and Desains £ confirm those hitherto referred to. Taking the 
absorbent power, as determined by these excellent experimenters, to express the radiating 
power, and multiplying their results by a common factor to facilitate comparison 
* And we also pass, as a general rule, from a series of bodies which vibrate in accord with the visible spec- 
trum to a series which vibrate in discord with the spectrum. The lowering of the rate of vibration is a con- 
sequence of chemical union. The comparative incompetence of compound bodies to oscillate in visual periods 
has incessantly declared itself in these researches. I would here refer to a most interesting illustration of the 
same kind, derived from the experiments of MM. De la Pkovostate and Desains. These distinguished experi- 
menters were the first to record the important fact that the qualities of heat emitted by bodies at the same tem- 
perature may be very unlike. Two experiments illustrate this fact. The first is recorded in the Comptes Rendus, 
vol. xxxiv. p. 951. One half of a cube was coated with lampblack, and the other half with cinnabar. The cube 
being filled with oil at a temperature of 173° C., it was found that the emission from the cinnabar was more 
copiously absorbed by a plate of glass than that from the lampblack. In the second experiment, they found that, 
while 39 per cent, of the radiation from a bright surface of platinum was transmitted by a plate of glass, only 
29 per cent, of the radiation from the opposite surface of the same plate, which was coated with borate of lead, 
was transmitted. These results are quite in harmony with the views which I have ventured to enunciate. We 
may infer from them that the heat emitted by the respective compounds — the cinnabar and the borate of lead — 
is of slower period than that emitted by the elements ; for experiment proves that as the periods are quickened 
the glass becomes more transparent. At a temperature of 100° C., moreover, the emission from borate of lead 
was found equal to that from lampblack (Comptes Rendus, vol. xxxviii. p. 442), while at a temperature of 550° C. 
it had only three-fourths of the emissive power of the lampblack. With reference to the theoretic views which 
these researches are intended to foreshadow, the results of MM. De la Peovostaye and Desains are of the highest 
interest. 
f Comptes Rendus, 1846, vol. xxii. p. 1139. + Annales de Chimie, 1850, vol. xxx. p. 442. 
MDCCCLXIV. 3 D 
