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y. On the Absorption and JRadiation of Heat by Gaseous Matter. — Second Memoir. By 
JoHX Ty^^^dall, F.E.S., Member of the Academies and Societies of Holland, Geneva, 
Gottingen, Zurich, Halle, Marburg, Breslau, Upsala, la Societe Philomathigue of 
Paris, &c. ; Professor of Natural Philosophy in the Boyal Institution. 
Eeceived January 9, — Eead January 30, 1862. 
^ 1. Instruments. 
The apparatus made use of in this inquiry is the same in principle as that employed in 
my last investigation*. It grew up in the following way: — A tube was first procured 
to receive the gases through which radiant heat was to be transmitted, but it was neces- 
sary to close the ends of this tube by a substance pervious to all kinds of heat, obscure 
as well as luminous. Eock-salt fulfils this condition, and accordingly plates of the sub- 
stance an inch in thickness, so as to be able to endui’e considerable pressure, were 
resorted to. In the earliest experiments a cube of boiling water was placed before one 
end of this tube, and a thermo-electric pile connected with a galvanometer at the other ; 
it was found that if the needle pointed to any particular degree when the tube was 
exhausted, it pointed to the same degree when the tube was filled with air. Thus 
tested, the presence of dry air, oxygen, nitrogen, or hydrogen had no sensible influence 
on the radiant heat passing through the tube. 
In some of these trials the needle stood at 80°, in some at 20°, and in others at inter- 
mediate positions. I reasoned thus : — The quantity of heat which produces the deflec- 
tion of 20° is exceedingly small, and hence a minute fraction of this quantity, even if 
absorbed, might well escape detection. On the other hand, the quantity of heat which 
produces the deflection of 80° is comparatively large, but then it would require a large 
absorption to move the needle even half a degree in this position. A deflection of 
20° is represented by the number 20, but a deflection of 80° is represented by the 
number 710. 'SVliile pointing to 80, therefore, an absorption capable of producing a 
deflection of 15 or 20 degrees on the lower part of the scale, would hardly produce a 
sensible motion of the needle. The problem then was, to work with a copious radiation, 
and at the same time to preserve the needle in a position where it would be sensitive to 
the slightest fluctuations in the absolute amount of heat falling upon the pile. 
This problem was finally solved by converting the pile into a difierential thermometer. 
Its second face was exposed, and a second source of heat was placed in front of that 
face. A moveable screen wus interposed between the two, by the motion of which the 
same amount of heat could be caused to fall upon the posterior surface of the pile as 
* Philosophical Transactions, 1861. 
