50 DYNAMIC ABSORPTION AND RADIATION. PART i. 



out, very dry atmospheric air was introduced into the 

 tube, the needle pointed to 59 indicating absorption ; 

 and when it was pumped out again the needle swung to 

 nearly 40 on the other side of zero, indicating radiation. 

 Remembering that the radiation and absorption of dry 

 air only produce a deflection of 1, it is evident that the 

 preceding great deflection of the needle is entirely owing* 

 to the action of the small residue of olefiant gas that 

 remained in the exhausted tube. In order to ascertain 

 how much the quantity of a gas or vapour might be 

 reduced before its action became insensible, the vapour 

 of boracic ether, which has the greatest absorptive 

 energy, was chosen. 



The mercurial gauge for measuring the pressure or 

 tension of the vapour already mentioned remained at- 

 tached to the apparatus. When one-tenth of an inch 

 of the vapour of boracic ether was admitted into the 

 exhausted tube, the barometer stood at 30 inches : hence 

 the tension of the vapour within the tube was the -g-J-oth 

 part of an atmosphere. Dynamically heated by dry air 

 the radiation of the vapour produced a deflection of 56. 

 Again the tube was exhausted to 0-2 of an inch and the 

 quantity of vapour was thereby reduced to f^-th of its 

 first amount; the needle was allowed to come to zero, 

 and the residue of the vapour produced a deflection of 

 42. The pump was again worked till a vacuum of 0'2 

 of an inch was obtained, this residue containing of 

 course the y^^th of the quantity of ether present in the 

 tube ; and on dynamically heating the residue, its radia- 

 tion produced a deflection of 20. 



Thus it is evident that the tension of the ether in 

 these experiments was continually diminished by the 

 0-2 of an inch, consequently its quantity was continually 

 diminished by its T 3-^th part; accompanied by a corre- 

 sponding decrease in the deflections of the needle. The 

 final result of this process showed that the radiation 



