488 Prof. Tyndall on the Action of Free Molecules on 



This, however, does not imply the acceptance of the infer- 

 ence from this inference, that the condensed films exert the 

 action ascribed to them on radiant heat. To test whether 

 they do so or not, I had a circular plate of rock-salt mounted 

 on its edge, and so placed that a beam of heat passed through 

 it normally to a distant thermopile. The source of heat was 

 an incandescent spiral, aided by a concave mirror ; and the 

 total radiation, after passing through the salt and impinging 

 on the pile, produced a deflection of 51°. This radiation was 

 accurately neutralized by a compensating cube, the needle 

 under the operation of the two opposing forces pointing to 0°. 

 By an arrangement* which every experimenter can imagine 

 for himself, sheets of air, laden with various vapours, could 

 be poured in succession over the plate of salt. If, under these 

 circumstances, absorbent films were formed, the equilibrium 

 would lie destroyed and the needle would move from 0°. My 

 slit, which was the flattened base of an open tin cone, was 

 placed across the upper part of the plate of rock-salt, so that 

 the vapours, which were all heavier than air, should fall over 

 the plate downwards. Here are the results obtained when 

 the following vapours were permitted to form films upon the 

 plate: — 



Bisulphide of carbon ... A barely sensible action. 



Chloroform No action. 



Benzol No action. 



Iodide of methyl .... No action. 



Iodide of ethyl No action. 



Amylene No action. 



Sulphuric ether A barely sensible action. 



Acetic ether No action. 



Formic ether No action. 



Alcohol No action. 



The minute deflections produced by bisulphide of carbon 

 and sulphuric ether had nothing to do with liquid films; so 

 that the words " no action " might have been written against 

 these substances as against the others. While therefore re- 

 cognizing the fact of condensation, these simple experiments 

 prove how incorrect it is to credit the condensed films with 

 the effects which have been ascribed to them. According to 

 Magnus, alcohol exhibits a force of vapour-hesion particularly 

 strong ; but we here see that even this vapour produces no 

 sensible effect. The vapours have been tested with other 

 sources of heat, with the same result, 



The deportment of dry and humid air is, as usual, very 

 instructive. When the thin plate of rock-salt resting on the 



