XIV. 

 PHYSICAL BASIS OF SOLAR CHEMISTRY* 



OMITTING all preface, attention was first drawn 

 to an experimental arrangement intended to 

 prove that gaseous bodies radiate heat in different de- 

 grees. Near a double screen of polished tin was placed 

 an ordinary ring gas-burner, and on this was placed 

 a hot copper ball, from which a column of heated air 

 ascended. Behind the screen, but so situated that no 

 ray from the ball could reach the instrument, was an 

 excellent thermo-electric pile, connected by wires with 

 a very delicate galvanometer. The pile was known to 

 be an instrument whereby heat is applied to the gen- 

 eration of electric currents; the strength of the current 

 being an accurate measure of the quantity of the heat. 

 As long as both faces of the pile are at the same tem- 

 perature, no current is produced; but the slightest 

 difference in the temperature of the two faces at once 

 declares itself by the production of a current, which, 

 when carried through the galvanometer, indicates by 

 the deflection of the needle both its strength and its 

 direction. 



The two faces of the pile were in the first instance 

 brought to the same temperature; the equilibrium 

 being shown by the needle of the galvanometer stand- 

 ing at zero. The rays emitted by the current of hot air 

 already referred to were permitted to fall upon one of 



* From a discourse delivered at the Royal Institution of Qreat 

 Britain, June 7, 1861. 



