XIV. 



PHYSICAL BASIS OF SOLAR CHEMISTRY. 1 



OMITTING- all preface, attention was first drawn to 

 an experimental arrangement intended to prove 

 that gaseous bodies radiate heat in different degrees. 

 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 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 tempera- 

 ture, 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 



1 From a discourse delivered at the lioyal Institution of Great 

 Britain, June 7, 1861. 



