RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
309 
I should be disposed to claim these experiments as telling in my favour. The first 
of them, in my opinion, dealt with the radiation not from dry air but from the 
adjacent aqueous vapour which had been warmed by the dry air. That the deflection 
in the second experiment was small is not surprising. The radiation which could 
reach the pile from a jet of air only 15 millimeters in diameter, and containing such 
moisture as could be taken up at 15° C., must have been extremely small under any 
circumstances. But in the present case, even this small radiation was diminished by 
the passage of the heat through 400 millimeters of undried air. I should demur to 
the explanation of the third experiment, and question the warrant to imagine a mist 
which could not be seen. Even the fourth experiment, where mist was visible, yielded, 
I doubt not, a mixed result; part of the effect, and probably the smallest part, being 
due to the mist, and part of it to the vapour. 
With regard to the radiation from hot aqueous vapour, the following experiment is 
typical of some hundreds which I have had occasion to make. A burner consisting of 
two rings provided with numerous small apertures was placed within a square tin 
chimney. At some height above the burner the chimney was perforated so as to 
enable the radiation from a heated gaseous column within the chimney to reach a distant 
thermopile. The side of the chimney facing the pile was so protected by screens that 
the radiation from the chimney itself was nil. Connecting the burner with a bottle 
of compressed hydrogen the gas was ignited. A column of hot vapour rose from, the 
burner and passed the aperture in the chimney, through which it sent its rays to the 
pile. Mere tips of flame were first employed ; the column of vapour rising from them 
sufficed nevertheless to produce a permanent deflection of 
40° 
A slight augmentation of the flame sent the needle up to 
60° 
A still further augmentation sent it up to 
75° 
This last deflection was equivalent to more than 400 of the degrees in the neighbour¬ 
hood of zero. 
The radiating column was here considerably above the flame. To examine the con¬ 
dition of the column a concentrated luminous beam was directed upon it. There was 
no precipitation. On the contrary, the suspended matter in the air of the chimney 
was much less than that of the surrounding air. Instead of a white mist, we had the 
blackness due to the destruction of the floating matter by the hydrogen flame. 
On quenching the flame the needle returned accurately to zero. 
In his objections Magnus, for the most part, dealt with true causes, but he erred as 
to their scope of action. I never denied the existence of the dangers which he empha¬ 
sized. The hygroscopic character of rocksalt, for example, to which he recurred so 
often, cannot be questioned. It has a strong attraction for moisture, especially when 
cold. On this point my experience has been large, and I applied it in the execution 
