f 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 laboured 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 90 deg.. This
experiment proved the vast difference 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 ebullition; and it was so arranged that the
quantities of heat falling from the cubes on the opposite faces of the
pile were exactly equal, thus neutralising each other. The needle of
the galvanometer being at zero, a sheet of oxygen gas was caused to
issue from a slit between one of the cubes and the adjacent face of
the pile. If this sheet of gas possessed any sensible power of
intercepting the thermal rays from the cube, one face of the pile
being deprived of the heat thus intercepted, a difference of
temperature between its two faces would instantly set in, and the
result would be declared by the ga