Radiation of Heat by Gaseous Matter. 343 



for the distant particles of olefiant gas. The length of the whole 

 column, in fact, might be available for the radiation of the 

 vapour, and a part of it only available for the gas. Cut off this 

 unavailable portion from the gas column, and we do not injure 

 its efficacy ; but cut off a similar length from the vapour column, 

 and we may materially diminish its effect. Speaking generally, 

 by reducing the column of ether and that of gas by the same 

 amount, the diminution of radiation will be most sensibly felt 

 where the radiant points are furthest asunder. Reasoning thus, 

 it becomes evident that in a long tube the vapour may excel the 

 gas in its amount of radiation, while with a short tube the gas 

 may excel the vapour. Let us now test this reasoning by ex- 

 periment. 



The dynamic radiation of the following four substances has 

 been tabulated thus : — 



o 



Sulphuric ether .... 64 



Formic ether 68*5 



Acetic ether 70 



Olefiant gas 63 



The action of olefiant gas is therefore smallest when the length 

 of the radiating column is 2 feet 9 inches. 



Experiments of the same character were made with a tube 

 3 inches long, or of the former length, and the following results 

 were obtained : — 



o 



Sulphuric ether . . . . 11 



Formic ether 12 



Acetic ether 15 



Olefiant gas 39 



The verification of the above theoretic reasoning is here com- 

 plete. It is proved that in a long tube the dynamic radiation of 

 the vapour exceeds that of olefiant gas, while in a short tube the 

 dynamic radiation of the gas far exceeds that of the vapour. 



§ 8. The apparatus with which these experiments were made 

 is capable of very diverse uses. Attached to a compression 

 pump, with it the relation between the mechanical force ex- 

 pended in compressing a gas and the heat developed might be 

 accurately determined. If oxygen, hydrogen, nitrogen, or air 

 were the body compressed, the conversion of vis viva into heat 

 might be declared by a modicum of vapour always kept in the 

 tube, while a compound gas would tell its own tale. 



Another interesting point might be, and indeed has been, 

 settled by the apparatus. Some years ago a discussion was car- 

 ried on between Professors Challis and Stokes on Laplace's cor- 

 rection for the velocity of sound, Professor Challis contending 



