30 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
The heated products of combustion acted on the pile in the above experiments, but 
the radiation from pure air was easily demonstrated by placing a heated iron spatula 
or metal sphere behind the screen. A deflection was thus obtained, which, when the 
spatula was raised to a red heat, amounted to more than sixty degrees. This action was 
due solely to the radiation of the air; no radiation from the spatula to the pile was 
possible, and no portion of the heated air itself approached the pile so as to communi- 
cate its warmth by contact to the latter. These effects are so easily produced, that I 
am at a loss to account for the inability of so excellent an experimenter as Melloni to 
obtain them. 
My next care was to examine whether difierent gases possessed diflerent powers of 
radiation, and for this purpose the following arrangement was devised. P in the woodcut 
represents the thermo-electric pile with its two conical reflectors ; S is a double screen 
of polished tin ; A is an argand burner consisting of two concentric rings perforated 
with oriflces for the escape of the gas ; C is a heated copper ball ; the tube 1 1 leads 
to a gas-holder containing the gas to be examined. When the ball C is placed on the 
argand burner, it of course heats the air in contact with it ; an ascending current is 
established, which acts on the pile as in the experiments last described. It was found 
necessary to neutralize this radiation from the heated air, and for this purpose a large 
Leslie’s cube L, filled with water a few degrees above the temperature of the air, was 
allowed to act on the opposite face of the pile. 
When the needle was thus brought to zero, the cock of the gas-holder was turned 
on ; the gas, passed through the burner, came into contact with the ball, and ascended 
afterwards in a heated column in front of the pile. The galvanometer was now observed, 
and the limit of the arc through which its needle was urged was noted. It is needless 
to remark that the ball was entirely hidden by the screen from the thermo-electric pile ; 
