52 DYNAMICAL EVOLUTION OF HEAT. PART i. 



All sources of heat being dispensed with, the chamber 

 next the Tfiterc pile contained the gas which was to act 

 as an absorber, and the more remote as a radiator. 



Heat is evolved in air when its motion is arrested ; on 

 entering an exhausted tube, the more rapid the motion 

 the greater the heat. Both chambers of the cylinder 

 were at first filled with the vapour to be examined, the 

 usual pressure being the -^ part of an atmosphere. 

 But the vapour entered so slowly, and the quantity was 

 so small that the radiation due to the warming of the 

 vapour by its own collision was insensible. The needle 

 of the goniometer being at zero, dry air was allowed to 

 enter the chamber most distant from the pile ; this air 

 became heated dynamically by the collision of its par- 

 ticles against the sides of the tube, communicated its 

 heat to the vapour, and the vapour immediately dis- 

 charged the heat thus communicated to it against the 

 pile. This case not only resembles, but is actually of 

 the same mechanical character as, that in which a 

 vibrating tuning fork is brought into contact with a 

 surface of some extent. The fork, which before was 

 inaudible, becomes at once a copious source of sound. 

 What the sounding board is to the fork, the compound 

 molecule is to the elementary atom. The tuning fork 

 vibrating alone is in the condition of the atom radiating 

 alone ; the sound of the one and the heat of the other 

 being insensible. But in association with sulphuric 

 or acetic ether vapour the elementary atom is in the 

 condition of the tuning fork applied to its sounding- 

 board, communicating motion to the luminiferous ether 

 through the molecules, as the fork through the board 

 communicates its motion to the air. 



Mr. TyiidalPs experiments show the great opacity of 

 a gas to radiations from the same gas, and may like wise 

 show the remarkable influence of attenuation in the 

 case of vapour. The individual molecules of a vapour 



