440 Prof. Tyndall on the Action of Free Molecules on 



was to illustrate the hold which experiment had obtained of a 

 subject previously considered intractable. The densities of 

 the gases and vapours employed were therefore varied within 

 -wide limits. In the experimental tube first made use of, a 

 full atmosphere of defiant gas absorbed more than 80 per 

 cent, of the entire radiation; and it was therefore evident that 

 a small fraction of an atmosphere of such gas would exert a 

 measurable action. On trial, it was found possible to measure 

 the absorption of j^^ of an atmosphere of olefiant gas. The 

 action of this gas was determined at sixteen different densities, 

 the absorption, as long as the density was very small, being 

 accurately proportional to the quantity of gas present . Similar 

 experiments were made, and similar results obtained with 

 other gases. The action of sulphuric ether vapour upon 

 radiant heat was proved to be still more powerful than that of 

 olefiant gas. The vapour was first carried into the experi- 

 mental tube by a current of dry air : and the pure vapour 

 was afterwards examined at seventeen different densities. 

 Bisulphide of carbon was tested at twenty different densities. 

 amylene at ten. benzol at twenty, and so of the others. 



Considering the views previously entertained regarding the 

 diathermancy of gases and vapours, I was naturally impressed 

 with these results. Sceptical when I first observed them, I 

 scrutinized them closely, until repeated scrutiny abolished 

 every 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 connecting 

 the exhausted experimental tube with a holder containing 

 a powerful gas, the needle would fly aside, owing to the 

 .-roppage 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 a priori ground 

 for supposing that its action would prove insensible : for win- 

 should T assume that T| ! )() 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 \ worked, and found its 



