52 Prof. Tyndall on the Passage of Radiant Heat 



temperature be the mean temperature of the mass of the pile, it 

 will chill the upper face and warm the lower one at the same 

 time. No matter, then, what the temperature of the air may be 

 when it enters the vessel of Prof. Magnus, the effect of its con- 

 tact with the pile is to diminish the deflection due to the radia- 

 tion from the source, and thus produce the same galvano- 

 metric effect as a true absorption would produce, the effect, how- 

 ever, being one of pure contact, in which absorption has abso- 

 lutely nothing to do. How Prof. Magnus releases his pile from 

 this apparently inevitable action he does not inform us ; and how 

 he can distinguish between this effect and one of absorption I 

 am at a loss to imagine. 



His apparatus will enable him to make this experiment in a 

 far more unexceptionable manner. Let him place a second 

 plate of salt across his tube at E E, and thus isolate his pile 

 from the air which he intends to examine. He will then obtain 

 the almost pure effect of radiation. Prof. Magnus has actually 

 made this experiment, and the result, expressed in his own 

 words, is " a hardly perceptible difference between dry air and 

 vacuum." 



It is scarcely necessary to repeat what I have already state 

 regarding the heating of the pile when the air enters, in virtue 

 of the collision of the air-particles against the face of the pile. 

 Curiously enough Prof. Magnus never once refers to this effect, 

 though he does refer, for the first time, in his last paper to the 

 chilling consequent on pumping out. I think it is almost certain 

 that, had his apparatus been sufficiently delicate, the striking 

 effect to which I refer must have long ago attracted his atten- 

 tion. Some conception of its magnitude may be formed from 

 the following quotation from a paper laid before the Royal 

 Society on the 18th of this month : — 



"A brass tube 3 feet long and very slightly tarnished within 

 was used for dynamic radiation. Dry air on entering the tube 

 produced a deflection of 12 degrees. The tube was then polished 

 within and the experiment repeated : the deflection by dry air 

 was instantly reduced to 7 # 5 degrees. 



"The rock-salt plate at the end of this tube was removed, and 

 a lining of black paper 2 feet long was introduced within it. 

 The tube was again closed, and the experiment of allowing dry 

 air to enter it was repeated. The deflections in three successive 

 experiments rose from 7°*5 to 



80°, 81°, 80°, 

 and this result might be obtained as long as the lining was per- 

 mitted to remain within the tube. 



" The plate of rock-salt was again removed, and the length of 

 the lining was reduced to a foot; the dynamic radiation on the 



