y 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 sijuare of glass 

 was placed between a thermometer and the lieat of a lighted candle 

 concentrated by the apparatus ; the mercury in some moniejits 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 verj' 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 f. Now it is quite evident that, in this case, a successive propa- 

 gation through the several layers of the sci'een, which was in a state of 

 perpetual change, could not take place. It was admitted, therefore, 

 that other diaphanous media besides atmospheric air sometimes transmit 

 the rays of heat as instantaneously as thej' 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 pi'oblem by employing a method invented by MaycockJ. 



* Pictet, Essai sur le Feu, § 52 et scq. 



t Journal de Phijs^que, de Chiniie, d'Histoire Noturelle et des Arts, par M. 

 Delamethevie, 1811. — P. Prevost, Memoire sur la Transmissio?t du Caloriqtie a 

 travers I'Eau et d'autrcs Substances, § 42 et 43. 



X Nicholson, A Jounial of Natural Philosophy, Cliemistry and the Arts, 

 vol. xxvi. May and June 1810. — J. D. Maycock, Remarks on Professor Leslie's 

 Docti-ine of Radiant Heat. 



