258 Prof. Tyudall on Recent Researches on Radiant Heat. 



more than a year, is simply to be ascribed to the fact of my 

 attention having been directed to the radiation of heat through 

 gases long before even his researches on conduction had com- 

 menced. It is needless to dwell upon the value of such a general 

 corroboration as that which subsists between Prof. Magnus and 

 myself. However private interests may fare, science is assuredly 

 a gainer when independent courses of experiment lead, as in 

 the present instance, to the same important results. 



§ 3. But while furnishing, by an independent method, a 

 highly valuable general corroboration of my results, there are 

 some special points on which Prof. Magnus differs from me; 

 and one of these (the action of aqueous vapour on radiant heat) 

 he has made the subject of special examination. My first 

 experiment gave the action of the vapour of the London air on 

 a November day to be 15 times that of the air itself. Only a 

 few weeks subsequently Prof. Magnus announced, and cited very 

 clear experiments in support of his statement, that the amount 

 of aqueous vapour capable of being taken up by air at a tempe- 

 rature of 15° C. has no influence whatever upon the absorption. 

 This announcement caused me to repeat my experiments with 

 more than usual care; and I found the absorption of the vapour 

 not 15 times, but 40 times that of the air. This result was 

 mentioned incidentally in my letter to Sir John Herschel ; and 

 Prof. Magnus, induced by this mention to take up the question 

 again, corroborates his former result, and finds, by repeated 

 experiments, that the aqueous vapour of the atmosphere has no 

 influence whatever upon radiant heat, " and that the rays of the 

 sun, so long as the air is clear, reach the earth in the same man- 

 ner whether the atmosphere is saturated with vapour or not.'" 



The more I experiment, the further I seem to retreat from 

 the position of my friend ; for in a paper quite recently presented 

 to the Royal Society, I have set down the action of the air of 

 the laboratory of the Royal Institution, not at 15, nor at 40, 

 but often at 60 times that of perfectly dry air. In fact, the 

 more experienced I become, and the greater the precautions I 

 take to exclude impurities, the more does atmospheric air, in its 

 action on radiant heat, approach the character of a vacuum, and 

 consequently the greater, by comparison, becomes the action of 

 the aqueous vapour of the air. 



In the paper which has suggested this communication, Prof. 

 Magnus assigns as a possible source of error on my part, 

 that the aqueous vapour may have been precipitated in a liquid 

 form upon my plates of rock-salt. He cites experiments 

 of his ow T n to show the hygroscopic nature of this substance ; 

 and refers to Melloni's experiments in proof of the highly 

 opake character of a solution of rock-salt for the obscure 



