PHYSICAL BASIS OF SOLAR CHEMISTRY. 251 



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 witli a very delicate galvanometer. 

 The pile was known to be an instrument whereby heat is 

 applied to the generation 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 temperature, 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 standing at zero. 

 The rays emitted by the current of hot air already referred 

 to were permitted to fall upon one of the faces of the pile; 

 and an extremely slight movement of the needle showed 

 that the radiation from the hot air, though sensible, was 

 extremely feeble. Connected with the ring-burner was a 

 holder containing oxygen gas; and by turning a cock, a 

 stream of this gas was permitted to issue from the burner, 

 strike the copper ball, and ascend in a heated column in 

 front of the pile. The result was, that oxygen showed 

 itself, as a radiator of heat, to be quite as feeble as 

 atmospheric air. 



A second holder containing olefiant gas was then 

 connected with the ring-burner. Oxygen and air had 

 already flowed over the ball and cooled it in some degree. 

 Hence the olefiant gas labored under a disadvantage. But 

 on permitting the gas to rise from the ball, it casts an 

 amount of heat against the adjacent face of the pile 

 sufficient to impel the needle of the galvanometer almost 

 to ninety degrees. This experiment proved the vast dif- 

 ference between two equally invisible gases with regard to 

 their power of emitting radiant heat. 



The converse experiment was now performed. The 

 thermo-electric pile was removed and placed between two 

 cubes filled with water kept in a state of constant ebulli- 

 tion; and it was so arranged that the quantities of heat 

 falling from the cubes on the opposite faces of the pile 



