Radiant Heat, and its Conversion thereby into Sound. 451 



action, and yet no reference is made to it. Either, then, the 

 disturbance was overlooked, or the apparatus was not suffi- 

 ciently delicate to reveal it. To these two sources of dis- 

 turbance (the lowering of the temperature of the source by 

 convection, and the warming and chilling of the pile by 

 contact, condensation, and evaporation) is to be added another, 

 due to the warming which must have occurred when the 

 dynamically heated air came into direct contact with the 

 thermopile — an action which, in my apparatus, proved suffi- 

 cient to whirl the needle of the galvanometer more than once 

 through an entire circle. 



Magnus next experimented with glass tubes 1 metre long 

 and closed at the ends with plates of glass. His source of 

 heat was a strong gas-flame aided, as in the experiments of 

 Dr. Franz, by a parabolic mirror. Two tubes were employed, 

 the one blackened within, and the other unblackened. With 

 the blackened tube an absorption of 2*44 per cent, was found 

 for air, and an absorption of 3*75 per cent, for hydrogen. In 

 the unblackened tube the absorption by air was 14*75 per 

 cent., and by hydrogen 16*27 per cent, of the total radiation. 



I went over this ground "with the utmost care, using in- 

 visible as well as visible heat. But, substituting plates of 

 pellucid rock-salt for the plates of glass, I failed to realize 

 the effect obtained by Magnus. He ascribed the difference 

 between the results obtained with his blackened and his un- 

 blackened tube to a change of quality in the heat, produced 

 by reflection from the interior surface of the latter. With 

 plates of rock-salt, however, though the reflection abides, the 

 change of quality does not occur. My position, therefore, in 

 regard to these experiments is similar to what it was in regard 

 to those of Dr. Franz. The results obtained with air, oxygen, 

 and hydrogen were, I hold, due to the chilling of the heated 

 glass ends of the tube by the cold gases, and the consequent 

 lowering of the secondary radiation. 



It was shown by Magnus himself, and is moreover obvious 

 at first sight, that the unblackened tube sent a far greater 

 amount of heat to the glass plate adjacent to the thermopile 

 than the blackened one. That plate being more heated by 

 the source, was more chilled by the air when it entered. The 

 greater cooling-power of hydrogen accounts, moreover, for 

 the advance of the supposed absorption from 14*75 to 16*27 

 per cent. With carbonic acid Magnus detected a difference 

 which had escaped Dr. Franz. Instead of making the action 

 of this gas equal to that of air, he found in the blackened 

 tube an absorption of 8*19 per cent., and in the unblackened 

 tube an absorption of 21*92 per cent., exerted by carbonic acid. 



