EADIATION OF HEAT BY GASEOUS MATTEE. 
79 
I also satisfied myself of the energetic radiation of the two following gases, which, how- 
ever, were used in irregular quantities. They were admitted into the tube from a large 
bolthead, until a common tension was established between the gas in the tube and the 
gas in the bolthead. 
Eadiation. Absorption. 
O O 
Ammonia 15 in. tension .... 56’5 33‘5 
Sulphurous acid 16 in. tension . . 45 24 
Let us reflect for an instant on the condition of our tube with its ^ inch of vapour 
at the moment when the latter has been heated by the entrance of the air. The 
gaseous column is heated throughout to the same temperature ; the elastic condition of 
the luminiferous ether is the same for all the particles, and consequently their periods 
of vibration are all the same. Hence each molecule is in that precise condition which 
enables it to absorb most effectually the undulations emanating from its neighbours. 
The rays from the particles at the end of the tube most distant from the pile have to 
cross a space of nearly 3 feet before they reach the latter, this space being partially 
filled with molecules circumstanced as just described. Hence absorption to a compara- 
tively greater extent must occur- ; and indeed we can imagine the tube so long that its 
frontal portion should furnish a vapour screen absolutely opake to the radiation of its 
hinder portion. Comparing ether vapour with olefiant gas, it is, I think, evident that the 
radiant points of the attenuated vapour which depresses the mercury column only 0-5 of 
an inch, are further apart than those of the gas which depresses the column 30 inches. 
Consequently there is a wider door open for the radiation of the distant ether particles 
towards the pile than 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. Eeasoning 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 experiment. 
The dynamic radiation of the folloA\fing 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 
