54 On the Passage of Radiant Heat through Dry and Humid Air. 



rock-salt. This is admitted. His experiments on this sub- 

 stance are quite correct ; but they have no bearing upon mine. 

 During our joint experiments, and while the humid air, whose 

 absorption produced a deflection of 43 degrees of my galva- 

 nometer, was still in the experimental tube, the rock-salt plates 

 were detached and placed in the hands of Prof. Magnus. He saw 

 no moisture, and he expressed himself satisfied that there was 

 none. I may add that there is the strongest reason to believe 

 that the substance as a liquid film, even if such existed, would 

 not exert any greater action than the same film as vapour. 

 However, the film did not exist, and it is therefore useless to 

 speculate about it. Prof. Magnus finds another difficulty in the 

 fact that I make air my unit, and refer the action of all other 

 gases to this unit. There is, I submit, no more " difficulty " 

 here than in the tables of atomic weights, where hydrogen is 

 taken as the unit. My object was, and is, to make radiant heat 

 an explorer of molecular condition ; and my results seem to me 

 more instructive and emphatic as I presented them than if I 

 had followed the common method pursued by Prof. Magnus. 

 The difficulty referred to does not touch the method of ex- 

 periment at all, but merely my way of presenting the results 

 obtained by that method. 



In conclusion, I would refer the reader, for additional proofs 

 of the action of atmospheric vapour upon radiant heat, to a reprint 

 of a short memoir from the Philosophical Transactions in the pre- 

 sent Number of the Philosophical Magazine. I will not dwell upon 

 this paper, as the competent reader will draw his own conclusions 

 from it. I may add that, in the paper presented to the Royal 

 Society last Thursday, the action of all the vapours which I have 

 examined is compared with that of the liquids from which these 

 vapours are derived. The order of absorption of vapours and 

 liquids is precisely the same. At the bottom of the list stands 

 water, as the most opake liquid examined. It would form a 

 most remarkable exception to what, so far as I can see, is a 

 general law, if the vapour of this liquid proved so ineffectual as 

 the experiments of Prof. Magnus make it to be. One word 

 with reference to the importance of this subject. In a certain 

 sense Prof. Magnus is quite right in rating it low. It derives 

 its importance from the accident that aqueous vapour is every- 

 where present in our atmosphere, and from the fact that, for the 

 future, the proved action of this vapour must form one of the 

 chief foundation-stones of the science of meteorology. 



Royal Institution, 

 June 19, 1863. 



