252 Prof. Tyndall on Recent Researches on Radiant Heat. 



even when the outer sides of the plates of rock salt were simul- 

 taneously kept moist. 



I do not venture to maintain that the remarkable results 

 which Dr. Tyndall obtained on the 10th of October of this year, 

 depend on the hygroscopic properties of his plates of rock salt, 

 since I neither know sufficiently the quality of these plates, nor 

 the precautions which Dr. Tyndall took in his experiments. My 

 only object is to call attention to the difficulties incidental to the 

 use of plates of rock salt in such experiments. 



XXXV. Remarks on Recent Researches on Radiant Heat. 

 By John Tyndall, F.R.S.* 



§ 1. HT^HE last Number of PoggendorfFs Annalen contains a 

 JL short paper by Professor Magnus, "On the Passage 

 of Radiant Heat through moist air," a translation of which 

 appears in the present Number of the Philosophical Magazine. 

 This paper has excited considerable interest and some dis- 

 cussion among the scientific men of London, and it is on many- 

 accounts desirable that I should not delay attempting to offer 

 an explanation of the differences which exist between my emi- 

 nent friend and myself. A brief sketch of the history of the sub- 

 ject is also considered desirable ; and this, as far as the extremely 

 limited time at my disposal will admit of, I shall also endeavour 

 to supply. 



§ 2. On the first perusal of Melloni's admirable work La 

 Thermochrose, which came into my hands soon after its publica- 

 tion, the thought of investigating the action of gases on radi- 

 ant heat occurred to me. Melloni, it will be remembered, failed 

 to obtain any evidence of the absorption of radiant heat by 

 a column of atmospheric air 18 or 20 feet long. My attention 

 was further fixed upon this subject by the discussion carried on 

 in 1851 between Professors Stokes and Challis, regarding 

 Laplace's correction for the theoretic velocity of sound. Pro- 

 fessor Challis, it will be remembered, contended that Laplace 

 had no right to his correction, because the heat evolved in con- 

 densation would be instantly wasted by radiation in a mass of 

 air of indefinite extension. In the first lecture of my first 

 course at the Royal Institution in 1853, I proposed compress- 

 ing air in a rock-salt syringe to decide the question; and in 

 a paper presented quite recently to the Royal Society, I have 

 solved this point in a manner which I hope Professor Challis 

 himself will deem conclusive, the mode of solution resembling in 

 some respects my device of 1853. In 1854 the action of gases 



* Communicated by the Author. 



