354 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
vapour and carbonic acid. Highly heated nitrogen is also present, which may produce 
a sensible effect : the unburnt gas, moreover, in proximity with the flame, and warmed 
by it, may contribute to the radiation, even before it unites with the atmospheric oxygen. 
But the main source of the radiation is, no doubt, the aqueous vapour and the carbonic 
acid. I wished to separate these two constituents, and to study them separately. The 
radiation of aqueous vapour could be obtained from a flame of pure hydrogen, while 
that of carbonic acid could be obtained from an ignited jet of carbonic oxide. To me 
the radiation from the hydrogen-flame possessed a peculiar interest ; for notwithstand- 
ing the high temperature of such a flame, I thought it likely that the accord between its 
periods of vibration and those of the cool aqueous vapour of the atmosphere would still 
be such as to cause the atmospheric vapour to exert a special absorbent power upon the 
radiation. The following experiments test this surmise : — 
Table XXVI. — Radiation through Atmospheric Air. 
Deflection. 
O 
Dry air 0 
Undried air .... 2T5 
Total heat 60*4 
Source, a hydrogen-flame. 
Absorption. 
0 
17-20 
100-0 
Thus, in a polished tube 4 feet long, the aqueous vapour of our laboratory air absorbed 
17 per cent, of the radiation from the hydrogen-flame. A platinum spiral, raised by 
electricity to a degree of incandescence not greater than that obtainable by plunging a 
wire into the hydrogen-flame, was used as a source of heat ; of its radiation, the undried 
air of the laboratory absorbed 
5-8 per cent., 
or one-third of the quantity absorbed when the flame of hydrogen was employed. 
The plunging of a spiral of platinum wire into the flame reduces its temperature ; but 
it at the same time introduces vibrations which are not in accord with those of aqueous 
vapour : the absorption by ordinary undried air of heat emitted by this composite 
source amounted to 
8-6 per cent. 
On humid days the absorption of the rays emitted by a hydrogen-flame exceeds even 
the above large figure. Employing the same experimental tube and a new burner, the 
experiments were repeated some days subsequently, with the following result : — 
Table XXVII. — Radiation through Air. Source, hydrogen-flame. 
Absorption. 
Dry air 0 
Undried air .... 20*3 
The undried air here made use of embraced the carbonic acid of the atmosphere ; after 
the foregoing experiments, the air was conducted through a tube containing a solution 
