RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
323 
A column of humid air 38 inches long absorbs, according to these experiments, 
10'7 per cent, of the radiation from a hydrogen flame. I have been criticised for 
estimating the absorption of the earth’s rays within 10 feet of the earth’s surface at 
10 per cent. This estimate I consider a moderate one, and the foregoing experiments 
prove it to be so. 
It would be an error to suppose that determinations like these are easily made. 
They require the most scrupulous care for their successful accomplishment. The 
hydrogen issued from the gas holder through an orifice of fixed dimensions in a 
stream of the utmost possible constancy. It was then led into a Sugg’s regulator, 
whence it issued under an absolutely constant pressure. The flame issued from a 
circular brass burner with an aperture y-gths of an inch in diameter. It was carefully 
surrounded by a hoarding, the space within the hoarding being packed with horsehair. 
Every precaution, in fact, was taken to avoid the agitation of the air around the flame. 
Proper care was also taken to secure the pile against disturbance by air currents. 
The air being first purified, by passing it through caustic potash and sulphuric acid, 
was rendered humid by carrying it over wet bibulous paper contained in a suitable 
tube. It required some minutes to enter, and it was therefore necessary, by prior 
patient observance of the needle, to make sure that during this interval no change 
occurred in the radiation save that effected by the humid air itself. The humid air 
was removed from the experimental tube, not by exhaustion which always causes 
precipitation, but by gently forcing, by means of a compressing pump, dry air through 
the tube. When this was done, the needle, in all the experiments above recorded, 
returned within a small fraction of a degree to zero.* 
With the rough, wide experimental tube to which reference has already been made, 
I, ten years ago, found the absorption of a column of humid air 38 inches long, to be 
8 per cent, of the total radiation from a flame of hydrogen. 
§. 6. Conservation of Molecular Action. 
If the absorption of radiant heat be the act of the constituent atoms of compound 
molecules, its amount depending solely on the number of molecules encountered by 
the calorific waves, then whatever may be the changes of density which gases and 
vapours undergo, so long as the number of molecules remains the same, the absorption 
ought to continue constant. Such constancy, should it be proved to exist, I name the 
“ conservation of molecular action.” The experiments now to be described deal with 
this question. 
Besides the silvered experimental tube already described as 38 inches long, I had 
another constructed of the same diameter, and with similar terminal apertures. Its 
length was 10 ‘8 inches. The one tube was, therefore, 3 A times the length of the 
* The temperature of the laboratory air daring the foregoing experiments was 60° Fah i;. 
2 T 2 
