526 On the Action of Free Molecules on Radiant Heat. 



fessor Langley, in the Sierra Nevada Mountains of California. 

 Professor Langley is known to have highly distinguished him- 

 self by researches on radiant heat, with instruments of his 

 own invention. He writes to me thus from Mount Whitney, 

 California, September 10, 1881 : — 



" I received your letter just as I was starting on the expe- 

 dition to this point of which I wrote. I much regretted that 

 I had not time to provide myself with your mercury-pyrhelio- 

 uieter; so I have been obliged to use the old form, with its 

 many disadvantages. 



" Our route here has led us through the dryest parts of this 

 continent, and across rainless deserts to this mountain, where 

 the air is perhaps drier than at any other equal altitude ever 

 used for scientific investigation. I write from an altitude of 

 12,000 feet, while the c Peak ' rises nearly 3000 more above 

 me. I have been successful in bringing up, and using here, 

 the rather complex and delicate apparatus for investigating the 

 absorption of the atmosphere on homogeneous rays, through- 

 out the visible and invisible spectrum. 



" You may be interested in knowing that the result indi- 

 cates 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 experiments also indicate an enormous extension 

 of the ultra-red spectrum beyond the point to which it has 

 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 every doubt as to the accuracy of the statements so 

 long since made by you, as to the absorbent power of this 

 agent over the greater part of the spectrum, and as to its pre- 

 dominant importance in modifying to us the solar energy. 

 " I am, with much regard, 



" Very truly yours, 



" S. P. Langley." 



