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 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 gal- 

 vanometer. The pile was known to be an instrument 

 whereby heat is applied to the generation of electric cur- 

 rents; the strength of the current being an accurate meas- 

 ure of the quantity of the heat. As long as both faces 

 of the pile are at the same temperature, no current is pro- 

 duced; 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. 



1 From a discourse delivered at the Royal Institution of Great Britain, June 

 7, 1861. 



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