THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



APRIL 1862. 



XXXIV. On the Passage of Radiant Heat through moist Air, and 

 on the Hygroscopic Properties of Rock Salt. By G. Magnus*. 



THE deportment of our atmosphere in reference to the passage 

 of the solar heat through it appeared especially important to 

 me in the investigation on the propagation of heat in gases, of 

 which I gave an account to the Konigliche Academie on the 30th 

 of July 1860f, and 7th of February 1861. Although it could be 

 foreseen that the small quantity of aqueous vapour which the air 

 can take up at the ordinary temperature would exercise a scarcely 

 perceptible influence on the passage of the thermal rays, yet, after 

 it had been made out that, under the same circumstances, oleflant 

 gas only transmits half as many rays as oxygen, and ammoniacal 

 gas still fewer, it appeared desirable to investigate whether that 

 anticipation was well founded or not. Experiment completely 

 confirmed it : neither by the application of a source of heat of 100° 

 nor by using a strong gas-flame could any difference be per- 

 ceived in the transmission of heat through air, whether it was dry 

 or saturated with vapour. This was the more surprising, inasmuch 

 as a paper by Dr. Tyndall J, " On the Absorption and Radiation 

 of Heat by Gases and Vapours," published simultaneously with 

 the above investigation, contained the statement that undried 

 atmospheric air, on a certain day, exhibited fifteen times as great 

 an absorption as dried air. Dr. Tyndall has subsequently been 

 further engaged on this subject, as appears from a letter to Sir 

 John Herschel§ recently published. In this Dr. Tyndall says, 

 a The results from which the opacity of the air has been in- 

 ferred are all to be ascribed to extraneous matters diffused in the 



* Translated from Poggendorff's Annalen, No. 12, 1861. 

 t Phil. Mag. vol. xx. j). 510. % Ibid. vol. xxii. p. 1G9. 



§ Ibid. vol. xxii. p. 377- 

 Phil. Mag. S. 4. Vol. 23. No. 154. April 1862. S 



