SECT. ii. DYNAMIC ABSORPTION AND RADIATION. 49 



must diminish and approximate to the absorption of 

 the sum as the two chambers become more unequal in 

 length, and that the sum of the absorptions of the two 

 chambers is a maximum when the medial plate of rock- 

 salt divides the long tube into two equal parts. 



When air enters an exhausted tube it is heated dy- 

 namically by the collision of its particles on the sides 

 of the tube as it rushes in to fill -the vacuum ; and when 

 the tube is exhausted again by the air pump, chilling is 

 produced by the application of a portion of the heat of 

 the air to generate vis viva. This dynamic principle 

 occurred in some of the experiments, and was dexte- 

 rously adopted and applied to the solution of a striking 

 and unprecedented problem: 'To determine the radia- 

 tion and absorption of gases and vapours without any 

 source of heat external to the gaseous body itself. 9 



The two external sources of heat being therefore dis- 

 pensed with in the absorptive apparatus, the thermo- 

 electric pile was presented to the cold glass tube which 

 was exhausted, and the needle of the goniometer stood 

 at zero. Nitrous acid on entering the exhausted tube 

 became heated and radiated its heat upon the adjacent 

 face of the pile which deflected the needle of the gonio- 

 meter through 28 in the direction that indicates ab- 

 sorption. As the heat of the gas became gradually 

 exhausted, the needle returned slowly to zero. The 

 pump was now worked, the rarefied gas in the tube was 

 chilled, and the adjacent face of the pile gradually 

 poured its heat on the chilled tube till the temperature 

 of the pile was so much lowered, that the needle was 

 deflected 20 on the negative side of zero,, that is on the 

 side denoting radiation. 



When olefiant gas entered the exhausted tube, the 

 needle showed an absorption of 67, and when the gas 

 was pumped out again, the needle showed a radiation 

 amounting to 41. When the gas was then pumped 



VOL. I. E 



