Radiant Heat to Aqueous Vapour, 33 



spheric aqueous vapour to several distinguished men, and among 

 others to Professor Magnus. After operating with common 

 undried air, which showed its usual absorption, and while the 

 undried air remained in the experimental tube, I removed the 

 plates of rock-salt from the tube and submitted them to the in- 

 spection of my friend. They were as dry as polished rock-crystal 

 or polished glass ; their polish was undimmed by humidity ; 

 and a dry handkerchief placed over the finger and drawn across 

 the plates left no trace behind it*. 



I would make one additional remark on the above experiments. 

 A reference to the plate which accompanies my two last papers 

 will show the thermo-electric pile standing, with its two conical 

 reflectors, at some little distance from the end of the experi- 

 mental tube. Hence, to reach the pile after it had quitted the 

 tube, the heat had to pass through a length of air somewhat 

 greater than the depth of the reflector. It has been suggested 

 to me that the calorific rays may be entirely sifted in this in- 

 terval — that all rays capable of being absorbed by air may be 

 absorbed in the space of air intervening between the experi- 

 mental tube and the adjacent face of the pile. If this were the 

 case, then the filling of the experimental tube itself with dry air 

 would produce no sensible absorption. Thus, it was imagined, 

 the neutrality of dry air which my experiments revealed might 

 be accounted for, and the difference between myself and Professor 

 Magnus, who obtained an absorption of 12 per cent, for dry air, 

 explained. But I think the hypothesis is disposed of by the 

 foregoing experiments; for here the reflector which separated 

 the pile from the tube no longer intervenes, and it cannot be 

 supposed that in an interval of ^th of an inch of air an absorp- 

 tion of 12 per cent, has taken place. If, however, a doubt on 

 this point should exist, I can state that I have purposely sent 

 radiant heat through an interval of 24 inches of dry air previous 

 to permitting it to enter my experimental tube, and found the 

 effects to be the same as when the beam had traversed 24 inches 

 of a vacuum. 



* The present Number of the Monatsbericht of the Academy of Berlin 

 contains an account of some experiments executed with plates of rock- 

 salt by Professor Magnus. The plates which stopped the ends of a tube 

 were so far wetted by humid air that the moisture trickled from them in 

 drops. As might be expected, the plates thus wetted cut off a large amount 

 of heat. The experiments are quite correct, but they have no bearing on 

 my results. In the earlier portions of my journal many similar cases are 

 described. In fact, it is by making myself, in the first place, acquainted 

 with the anomalies adduced by Professor Magnus, that I have been able to 

 render my results secure. I may add that the communication above referred 

 to was made to the Academy of Berlin before my friend had an opportunity 

 of examining my rock-salt plates. I do not think he would now urge this 

 objection against my mode of experiment. 



Phil. Mag. S. 4. Vol. 26. No. 172. July 1863. D 



