Radiation of Heat by Gaseous Matter, 



281 



such small quantities could be directly measured, the action of 

 air, oxygen, hydrogen, and nitrogen, in comparison with that of 

 the other substances at the same tension, would doubtless be 

 greatly reduced. With the energetic gases the rays are most 

 copiously struck down by the quantities which first enter the 

 tube, the quantities which enter last producing in many cases 

 an infinitesimal effect. Now I have shown in my last paper 

 that, for very small absorptions, the effect is sensibly proportional 

 to the quantity of gas present ; and this would seem to justify us 

 in assuming that for 1 inch of tension the absorption of air, 

 oxygen, nitrogen, and hydrogen would be ^th of the absorp- 

 tion at 30 inches. With all the other gases I have measured 

 directly the absorption of a quantity possessing in each case a 

 single inch of tension. Assuming the proportionality just re- 

 ferred to, and again calling the effect of air unity (the unit, 

 however, being only ^th of that in the last Table), the follow- 

 ing are the relative absorptions : — 



Table II. 



Air .... 



Oxygen . . . 



Nitrogen . . . 

 Hydrogen . . 

 Chlorine . . 

 Bromine . . 

 Hydrobromic acid 



1 



1 

 1 



1 



60 



160 



1005 



Carbonic oxide . . • 750 



Nitric oxide .... 1590 



Nitrous oxide . . . 1860 



Sulphide of hydrogen . 2100 



Ammonia .... 7260 



defiant gas . . . 7950 



Sulphurous acid . . 8800 



Here we have the extraordinary result, that, for tensions of 

 1 inch of mercury, the absorption of ammonia is over seven thousand 

 times, the absorption of olefiant gas seven thousand nine hundred 

 and fifty times, while the action of sulphurous acid is eight thousand 

 eight hundred times that of air. 



It is impossible not to be struck by the position of chlorine 

 and bromine in this Table. They are elements, and, notwith- 

 standing their colour and density, they take rank after the 

 transparent elementary gases. The perfectly transparent ole- 

 fiant gas absorbs more than one hundred and thirty times the 

 amount absorbed by the untransparent chlorine, and nearly fifty 

 times the quantity absorbed by the intensely brown vapour of 

 bromine. I canot think this fact insignificant. Hitherto che- 

 mists have spoken to us of elements, and we have helped our- 

 selves to conceptions regarding them and their compounds in 

 the only way possible to our mental constitution. But our con- 

 ceptions remained purely subjective, nor were we acquainted with 

 any physical trait which would in any degree justify these con- 

 ceptions. Here, however, we seem to touch the ultimate particles 



Phil Mag. S. 4. Vol. 24. No. 161. Oct. 1862. U 



