458 Prof. Tyndall on the Action of Free Molecules on 



vapours. More than this, the liquid layers were supposed to 

 be broken up into discontinuous patches, which not only ab- 

 sorbed the heat but scattered it. " Yapour-hesion," it may 

 be added, was found to vary with the liquid which produced 

 the vapour, being particularly strong in the case of alcohol. 



Magnus brought this generalization to the test of experi- 

 ment, but failed to verify it. He urged humid air against a 

 dry mirror, from which radiant heat was reflected; but unless 

 he wetted the mirror visibly, no effect was produced on the 

 reflected beam. Still he held that reflection, oft repeated, 

 rendered sensible an action which eluded a single reflection. 

 My position here is clear. I do not doubt surface attraction, 

 or deny the existence of impalpable films. No experiment 

 was ever made on the reflection of light or radiant heat in 

 which such films did not intervene; but they had as little effect 

 upon my results as they had upon those of De la Provostaye 

 and Desains*, and of other refined experimenters. As early 

 as 1859 I was made aware of the danger which might arise 

 from condensation. Warned by the action of chlorine on my 

 brass experimental tube, I coated it inside with lampblack, 

 and retested with it all my vapours. The result removed from 

 my mind the suspicion that surface condensation had any 

 thing to do with the observed absorption. Many similar ex- 

 periments with blackened tubes were subsequently made by 

 me, for my own safety and instruction. There was no sub- 

 stantial difference between the results obtained with such 

 tubes and those obtained with polished tubes in which internal 

 reflection came into play. 



Such are the general features and phases of a discussion 

 which, though dealing only with a small item of my work, 

 has consumed a considerable amount of time. Other able 

 experimenters have entered this field, the latest of whom, 

 MM. Lecher and Pernter, have published a long and learned 

 memoir in "Wiedemann's Annalen, which has been translated 

 in the Philosophical Magazine for January 1881. My expe- 

 riments with gases they corroborate, but not those with 

 vapours. Regarding the action of aqueous vapour they are 

 especially emphatic, their conclusion being "that moist air 

 does not perceptibly absorb the heat-rays from a source of 

 100° C." In fact, they found moist air a little more transpa- 



* Considering the energy of water as a radiator, exceeding, according 

 to Leslie, that of lampblack itself, the film of this liquid which must 

 have covered the plates of silver in the experiments of the two French 

 philosophers ought, if Magnus be correct, to have sensibly raised the 

 emission. Calling the emission from lampblack 100, that from polished 

 silver plus the film was only 2'1. 



