RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
307 
due to aqueous vapour, might be really due to the smoke and dust suspended in 
London air. To meet this I carried air myself from the Isle of Wight, had it 
carried from Epsom Downs and other places, and found the aqueous vapour diffused 
in such air to be from 60 to 70 times more energetic than the air itself London 
air, moreover, w T as freed from its suspended matter and tested when dry: it proved 
neutral. The self same air was then rendered humid: its absorbent power was 
restored. Then, with dry air as a carrier, I introduced smoke into the experimental 
tube, until it far exceeded in density that suspended in the London air when Magnus 
drew my attention to it. The quantity of heat intercepted by this smoke proved to 
be only a fraction of that absorbed by the perfectly invisible aqueous vapour. 
On his return to Berlin he resumed his labours. He had been especially impressed 
by the experiments with the open tube, and to this point he directed his chief atten¬ 
tion. “ The result of this experiment/’ he writes, “ was so surprising and so little in 
accord with what I had found by other methods, that on reaching home I determined 
to repeat the experiment.” He did so, with this result:—“I have,” he says, 
“ repeated the blowing in of dry air and moist air many hundred times; but in no 
single case was the deflection such as to indicate a greater absorption by moist air.” 
Humid air in his experiments produced the deflection of heat; dry air the deflection 
of cold—a result diametrically opposed to mine. In London he had seen that my 
deflections were as large as I had affirmed them to be, but he had not criticised them 
with a view of ascertaining whether they were or were not in the right direction. In 
these new experiments, however, he had, he thought, hit upon their origin. The 
moving air had reached the face of the thermopile, producing, when humid, heat by 
condensation, and when dry, cold by evaporation. 
I read the account of these experiments with some concern; for it was thereby 
made plain to me that Magnus had by no means realised the anxious care that I had 
bestowed upon my work. The testimony of an independent observer would, I thought, 
set the matter right. My apparatus, carefully adjusted, was accordingly handed over 
to Dr. Frankland, who minutely tested every point involved in, or arising out of, the 
objection of Magnus. He verified all my results. His opinion as to the accuracy of 
the method of compensation is worth recording. “In conclusion,” he writes, “I cannot 
but express my surprise and admiration at the precision and sharpness of the indica¬ 
tions of your apparatus. Without having actually worked with it I should not have 
thought it possible to obtain these qualities in so high a degree in determinations of 
such extreme delicacy.”! To this may be added the subsequent testimony of Professor 
* Poggendorff’s Annalen, 1863, vol. 118, p. 580; Phil. Mag., 1863, vol. 26, p. 25. 
t The total heat here employed amounted to of a quadrant. This exceedingly large deflection was 
neutralised by the radiation from the compensating cube. But so accurately were the two sources 
balanced, and so constant was the radiation on both sides, that the determinations were made with ease, 
and without sensible disturbance or fluctuation. 
2 ft 2 
