2 M. MELLONI ON THE FREE TRANSMISSION 
Pictet, however, corrected the mistake by means of the apparatus known 
by the name of conjugate mirrors. A very transparent square of glass 
was placed between a thermometer and the heat of a lighted candle 
concentrated by the apparatus; the mercury in some moments rose se- 
veral degrees ; there was a perceptible elevation of temperature also when 
the candle was removed and a small jar filled with boiling water put in 
its place *. 
Some years later Herschel undertook a very extensive series of ex- 
periments on the same subject. They are described in the volume of the 
Philosophical Transactions for 1800. The author employs no artifice 
to increase the action of the rays of heat, and contents himself with the 
direct measurement of their effect by placing the thermometer at a very 
short distance from the diaphanous body. 
But doubts were started as to the conclusions drawn from these dif- 
ferent results. It was objected that part of the radiant heat was first 
stopped at the nearer surface of the glass, that it was gradually accu- 
mulated there and afterwards propagated from layer to layer, until it 
reached the further surface whence it began again to radiate on the ther- 
mometer. It was maintained even that nearly the whole of the effect 
was produced by this propagation. In short, some went so far as to deny 
altogether that the heat emitted by terrestrial bodies can be freely trans- 
mitted through any other diaphanous substance than atmospheric air. 
M. Prevost, by means of a very ingenious contrivance, demonstrated 
the erroneousness of this opinion. Having attached to the pipe of a 
fountain a spout consisting of two parallel plates, he obtained a strip of 
water about a quarter of a line in thickness. On one side of this he 
placed an air thermometer and on the other a lighted candle or a hot 
iron. The thermometer rose, almost always, some fraction of a de- 
gree+. Now it is quite evident that, in this case, a successive propa- 
gation through the several layers of the screen, which was in a state of 
perpetual change, could not take place. It was admitted, therefore, 
that other diaphancus media besides atmospheric air sometimes transmit 
the rays of heat as instantaneously as they always transmit those of 
light. . 
M. Prevost’s process could not however be applied to solid bodies. It 
was therefore impossible to determine, by means of it, whether caloric was 
immediately transmitted through screens of glass. Delaroche completely 
solved this problem by employing a method invented by Maycocktf. 
* Pictet, Essai sur le Feu, § 52 et seq. 
+ Journal de Physique, de Chimie, d'Histoire Naturelle et des Aris, par M. 
Delametherie, 1811.—P. Prevost, Mémoire sur la Transmission du Calorique @ 
travers l’ Eau et d'autres Substances, § 42 et 438. 
¢ Nicholson, A Journal of Natural Philosophy, Chemistry and the Arts, 
vol. xxvi. May and June 1810.—J. D. Maycock, Remarks on Professor Leslie’s 
Doctrine of Radiant Heat. 
