RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
353 
emission, the ratio holding good up to the surface of the sun. But having placed, as 
I thought beyond doubt, the action of aqueous vapour on radiant-heat, and believing 
the action of the vapour to be substantially the same as that of water, I reasoned and 
experimented as follows in 1865 :—“The sun’s rays, before reaching our earth, have 
to pass through the atmosphere, where they encounter the atmospheric vapour which 
exercises a powerful absorption on the invisible calorific rays. From this, apart from 
other considerations, it would follow that the ratio of the invisible to the visible 
radiation in the case of the sun must be less than in the case of the electric light. 
Experiment, we see, justifies this conclusion. If we cause the beam from the electric 
lamp to pass through a layer of water of suitable thickness, we place its radiation in 
approximately the same condition as that of the sun; and on decomposing the beam, 
after it has been thus sifted, we obtain a distribution of heat closely resembling that 
observed in the solar spectrum.” 
If therefore, we could get above the vapour-screen which swathes the earth, the 
“powerful absorption” referred to in the paragraph just quoted would disappear, the 
ratio of the invisible to the visible solar rays being augmented correspondingly. That 
such would be the case I have long taken for granted, but I hardly hoped for a corro¬ 
boration so impressive as that furnished by the recent observations of Professor 
Langley, in the Sierra Nevada Mountains of California. Professor Langley is 
known to have highly distinguished himself by researches on radiant heat, with 
instruments of his own invention—he writes to me thus from Mount Whitney, 
California, Sept. 10, 1881 :— 
" I received your letter just as I was starting on the expedition to this point of 
which I wrote. I much regretted that I had not time to provide myself with your 
mercury pyrheliometer, so I have been obliged to use the old form, with its many 
disadvantages. 
“ Our route here has led us through the clryest 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 'Peak’ rises nearly 3,000 more above me. I have been suc¬ 
cessful in bringing up, and using here, the rather complex and delicate apparatus for 
investigating the absorption of the atmosphere on homogeneous rays, throughout the 
visible and invisible spectrum. 
“ You may 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 experiments also indicate an enormous extension of the ultra-red spectrum 
MDCCCLXXXII. 2 Z 
