Radiation of Heat by Gaseous Matter. 271 



this tube by a substance pervious to all kinds of heat, obscure as 

 well as luminous. Rock-salt fulfils this condition ; and accord- 

 ingly plates of the substance an inch in thickness, so as to be 

 able to endure 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 intermediate positions. I reasoned thus : — The 

 quantity of heat which produces the deflection of 20° is exceed- 

 ingly 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 com- 

 paratively 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. While pointing to 80, there- 

 fore, 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 

 differential thermometer. Its second face was exposed, and a 

 second source of heat was placed in front of that face. A move- 

 able screen was 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 that received from the experimental 

 tube by its anterior surface. When this was effected, no matter 

 how high the previous deflection might be, it was completely 

 neutralized, and the needle descended to zero. 



Supposing this equality to have been established when the 

 tube was exhausted, it is manifest that any gas, capable of ab- 

 sorbing even an extremely small proportion of radiant heat, 

 would, if introduced into the tube, destroy the equilibrium of 

 both sources. The second source of heat would now predomi- 

 nate, and a deflection of the galvanometer needle would be the 

 consequence. The magnitude of this deflection would depend 

 on the quantity of heat cut off by the gas, and, properly reduced, 

 it became a strict measure of the absorption. 



