452 Prof. Tyndall on the Action of Free Molecules on 



Here true absorption mixed itself with the effect of mere 

 chilling, while with still more powerful gases the effect of 

 chilling retreated hy comparison more and more. 



Such were the experiments which determined, in the first 

 instance, the attitude of this distinguished man towards that 

 portion of my work which related to the action of the air 

 and vapour of our atmosphere on radiant heat. In the defence 

 of his position he brought to bear all the resources of con- 

 summate skill and large experience. His position, however, 

 was by no means a wholly defensive one. He dwelt re- 

 peatedly and emphatically on the dangers — and they are real — 

 to which the method pursued by me was exposed. I had 

 closed my experimental tube with plates of transparent rock- 

 salt; and he urged against me the hygroscopic character of 

 this substance. Placing rock-salt beside a vessel of water 

 under a glass shade, he found that it could be rendered 

 dripping wet *. Hence his argument that, instead of 

 measuring the action of vapour, I had really measured the 

 action of brine. This, however, I could not admit. I was 

 aware of the danger, and had avoided it. In many hundred 

 instances the rock-salt plates had been detached from my 

 experimental tube while filled with the very air which had 

 produced the observed absorption, and found to be as dry as 

 polished plate-glass. For a week at a time I have charged 

 my experimental tube alternately with dried and undried air, 

 removing every evening the plates of salt while the humid 

 air filled the tube. Their dryness and polish were found 

 unimpaired f. I have frequently flooded the experimental 

 tube with light, and watched narrowly whether any dimness 

 showed itself on the salt, or on the interior surface, when the 

 humid air entered. There was nothing of the kind. I 

 finally abandoned the plates of salt altogether, and obtained 

 in a tube opened at both ends substantially the same effects 

 as those obtained when the tube was closed with plates of 

 rock-salt. 



In 1862 Magnus came to London. He had been previously 

 working at the points of difference between us, and had 

 strengthened his first conviction. The action of the air he 

 had found to be considerable, and the action of aqueous vapour 

 practically nil. I also had been working, but with an entirely 



* It has been shown by Professor De-war that the exposure of a dry 

 plate of rock-salt for five minutes to saturated air sensibly augments the 

 weight of the salt as determined by a delicate balance. 



t This mastery over the apparatus was not attained without training. 

 Any lapse of care soon declared itself by the condition of the plates of 

 salt. 



