RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
803 
increasing, fell to less than two-thirds of that of the shallower stratum. It is pretty 
obvious that an influence different from pure absorption came here into play. That 
influence was convection, 
Fig. 2. 
Anxious to probe this matter to the bottom, and to abolish, or account for, the 
differences between my friend and myself, I wrote to him proposing an exchange of 
apparatus—that he should send his to London, and I mine to Berlin. I afterwards 
had a fac-simile of his apparatus constructed in London, and satisfied myself by actual 
trial that it was really hampered with the defects I had ascribed to it. By means of 
the striae of incense smoke and of chloride of ammonium, the fact of convection in air 
was rendered plainly visible to the eye, while the behaviour of hydrogen, under like 
circumstances, revealed the cause of its transporting more heat than the vacuum in the 
first experiments, and less heat, not only than the vacuum, but than air or oxygen, in 
the second experiments. In the one case, the thermometer, being close to the source, 
came within the range of the convection currents of this mobile gas, the heat being 
transported to it by these currents. In the other case, a considerable distance inter¬ 
vened between the source and the pile, which was further effectually protected by the 
