296 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
doubt. For my own instruction I illustrated the action of the stronger gases and 
vapours in a variety of ways. Turning, for example, once rapidly round a cock con¬ 
necting the exhausted experimental tube with a holder containing a powerful gas, the 
needle would fly aside, owing to the stoppage of the heat by the infinitesimal amount 
of gas which entered the tube during the rotation. Discharging a powerful gas or 
vapour in free air, between the source of heat and the thermopile, a similar energetic 
action would be produced by the perfectly invisible agent. 
I was schooled in such actions before the thought of testing the omnipresent vapour 
of our atmosphere occurred to me. When it did occur there was in my mind no 
d 'priori ground for supposing that its action would prove insensible; for why should 
I assume that yj^th of an atmosphere of aqueous vapour would prove neutral, after I 
had proved a small fraction of this fraction, on the part of other gases and vapours, 
to be active ? There was no reason for such an assumption on my part—nothing to 
deter me from hopefully submitting the question to experiment. I accordingly tested 
the water vapour of the atmosphere in which I worked, and found its action on a first 
trial to be thirteen times that of the air in which it was diffused. 
It is not uninstructive to compare this approach to the problem with that of a very 
distinguished man—the late Professor Magnus, of Berlin. -5 ' Subsequent to me, he 
subjected the aqueous vapour of our atmosphere to an experimental test; but he 
made the experiment under the assured conviction that his result would be negative. 
“ It could,” he says, “ be foreseen with certainty that the small amount of aqueous 
vapour taken up by air at ordinary temperatures could exert no influence on the 
transmission.” I think it must be obvious that if Magnus had gone through the 
discipline to which I had been subjected, he would not have used this language. 
His mistake however was a natural one. In fact during the earlier stages of the 
inquiry my mind was exactly in the condition of his mind—I also thinking, until 
practically instructed to the contrary, that the action of aqueous vapour at ordinary 
temperatures must be immeasurably small. It is well known tha,t Magnus tested his 
foregone conclusion, and found it verified ; while I, on the other hand, as above stated, 
justified mine. 
The various gases which had been examined in the experimental tube with regard 
to their powers of absorption, were next tested as regards their powers of radiation. 
Columns of the heated gases were allowed to ascend in free air, and to radiate 
against the pile. In this simple way, the radiative power of “ transparent elastic 
fluids ” was for the first time established. The order in which the gases ranged them¬ 
selves, in regard to radiation, was exactly the order of their absorptions. Here, as in 
other cases, I instructed myself by observing how gases might be made to play the 
part of solids. Permitting, for example, a film of one of the stronger gases to glide 
* In many respects my generous and helpful friend, but, in regard to this question, my stead fas 
antagonist for many years, 
