38 Action of Free Molecules on Radiant Heat. [Xov. 24, 



" inaudible." In this connexion reference. was made to the interest- 

 ing experiments of Professor Rontgen, an independent and successful 

 worker in this field. 



In conclusion, the lecture draws attention to the bearing of its 

 results upon the phenomena of meteorology. The views of Magnus 

 regarding the part played by mist or haze, are referred to and atten- 

 tion is directed to various observations by Wells which are in opposi- 

 tion to these views. The observations of Wilson, Six, Leslie, 

 Denham, Hooker, Livingstone, Mitchell, Strachey, and others are 

 referred to and connected with the action of aqueous vapour upon 

 solar and terrestrial radiation. Many years ago the lecturer sought to 

 imitate the action of aqueous vapour on the solar rays by sending a 

 beam from the electric hffht through a laver of water, and afterwards 

 examining its spectrum. The curve representing the distribution of 

 heat resembled that obtained from the spectrum of the sun, the 

 invisible calorific radiation being reduced by the water from nearly 

 eight times to about twice the visible. Could we get above the screen 

 of atmospheric vapour, a large amount of the ultra-red rays would 

 assuredly be restored to the solar spectrum. This conclusion has 

 been recently established on the grandest scale by Professor Langley, 

 who on the 10th of September wrote to the lecturer from an elevation 

 of 12,000 feet on Mount Whitney, " where the air is perhaps drier than 

 at any other equal alticude ever used for scientific investigation." An 

 extract from Professor Langley' s letter will fitly close this summary: 

 — "You may," he says, "be interested in knowing that the result 

 indicates a great difference in the distribution of the solar energy here 

 from that to which we are accustomed in regions of ordinary humidity, 

 and that while the evidence of the effect of water vapour on the 

 more refrangible rays is feeble, there is, on the other hand, a 

 systematic effect, due to its absence, which shows, by contrast, its 

 power on the red and ultra-red in a striking light. These experi- 

 ments also indicate an enormous extension of the ultra-red rays 

 beyond the point to which they have been followed below, and being 

 made on a scale different from that of the laboratory — on one indeed 

 as grand as nature can furnish — and by means wholly independent of 

 those usually applied to the research, must, I think, when published, 

 put an end to any doubt as to the accuracy of the statements so long 

 since made by you, as to the absorbent power of water- vapour over 

 the greater part of the spectrum, and as to its predominant importance 

 in modifying to us the solar energy." 



