14 



Prof. J. Tyndall on 



[Dec. 18, 



enable the needle to rise from 0° to 28 o, 0, the temperature of the 

 presumptive source falling at the same time from 46° to 39° - 6. It is on 

 experiments conducted in this way that Professor Buff founds con- 

 clusions which, were they correct, would leave eight or nine years of 

 diligent and careful work, on my part, in a very serious plight. 

 Hydrogen gas he alleges to be as diathermanous as a vacuum, while 

 more than 50 per cent, of his radiation is, as before stated, absorbed 

 by 45 millims. of air. 



With the fact before him that the deflection of his galvanometer 

 needle was rising while the temperature of his brass plate was sinking, 

 it must have occurred to Professor Buff that not the brass but the 

 glass was his really efficient source of heat. The radiating zone was 

 chilled by his gases, currents were established, and that hydrogen 

 under such circumstances should transfer more heat to his junction 

 than air is only what might be expected from the mobility of the gas. 



Professor Buff next lowered his junction to a distance of 100 

 millims. below his source, and obtained an unexpected result. Instead 

 of the absorption increasing, as it ought to have done when the depth 

 of the absorbing layer was more than doubled, it fell from 50 to 15 per 

 cent. The deflections here were small, and on coating the polished 

 brass surface with lampblack, " though the radiating power of the 

 source of heat must have been six or seven times greater than previ- 

 ously," there was hardly any increase of the deflection. The junction 

 was here acted on simply by the chilled sides of the surrounding 

 vessel. I have failed to extract a clear meaning from Professor Buff's 

 remarks on this point ; but, as a matter of fact, when the interior of 

 his cylinder was covered with double cardboard, the absorptive energy 

 of dry air in great part reappeared. The only distinct impression here 

 left upon the mind is that, in Professor Buff's experiments, the 

 surface of his glass cylinder, which ought to have been absolutely 

 neutral, played a most influential part.* 



Experimental Examination of Professor Buff's Results. 



The disposition of the apparatus devised by me in 1859, when, in 

 relation to radiant heat, the gaseous form of matter was first brought 

 under the yoke of experiment,! is too well known to need description. 

 From the source of heat the rays passed first through a vacuous 

 chamber and then through an experimental tube stopped at both ends 



* Thinking that the original German might enable me to seize Professor Buff's 

 meaning more effectually, I sought for the original paper, but failed to find it. 

 The article appears to have been written and translated specially for the English 

 public. 



f Professor Magnus's first paper on radiation through gases was about a year and 

 three-quarters posterior to my first communication to the Royal Society. See " Con- 

 tributions to Molecular Physics, Historic Remarks on Memoir I," p. 59. 



