PHYSICAL BASIS OF SOLAR CHEMISTRY. 51 
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 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 witli 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 
