Prof. Tyndall on Recent Researches on Radiant Heat. 265 



of glass 2'6 millimetres in thickness. It is therefore almost 

 certain that 70 per cent, of the entire heat emitted by the lamp 

 of Prof. Magnus were lodged in his first glass plate. A much 

 less quantity of the direct heat would be absorbed by his second 

 plate ; but here the amount absorbed would be most effective as 

 a secondary source of heat, on account of the proximity of this 

 plate to the thermo-electric pile. 



With the blackened tube, then, we had three sources of heat 

 acting directly or indirectly upon the pile — the lamp, the first 

 plate of glass, and the second plate. In reality, however, the 

 sources reduce themselves to two. Por, glass being opake to 

 the radiation from glass, the heat emitted by the first plate was 

 expended in exalting the temperature of the second, close to 

 which the pile was placed. On admitting air at the ordinary 

 temperature into this tube, an effect similar in kind to that 

 which takes place in the other instrument of Prof. Magnus 

 must occur : the heated glass plates would be chilled, and they 

 would be chilled more by the hydrogen than by the air, thus 

 giving us the exact results recorded by Prof. Magnus. 



The same considerations applied to the unblackened tube, 

 explain perfectly the singular result obtained with it. On 

 theoretic grounds it is extremely difficult, if not impossible, to 

 conceive of such a change of quality as that above referred to. 

 But there appears to be no reason to call in its aid. Prof. 

 Magnus himself finds that the quantity of heat transmitted 

 through his unblackened tube is 26 times the quantity which 

 gets through his blackened one where the oblique radiation is 

 cut off. In the case therefore of the naked tube, the flux of 

 heat sent down by the heated glass plate adjacent to the lamp, 

 to its fellow at the other end, and likewise the heat sent directly 

 from the lamp to the same plate, are greatly superior to what 

 they are in the case of the blackened tube. The plate adjacent 

 to the pile becomes therefore more highly heated in the case of 

 the naked tube; arid as its chilling is approximately propor- 

 tionate to the difference of temperature between it and the cold 

 air, the withdrawal of heat will be greatest when the tube is 

 unblackened within. While leaving myself open to correction, 

 I would offer this as the explanation of the extraordinary result 

 which Prof. Magnus has obtained. It is, I submit, not a case 

 of absorption, but of direct chilling by the cold air. 



It is hardly necessary to say that similar remarks to those made 

 with reference to the blackened tube of Prof. Magnus apply to 

 the experiments of Dr. Pranz. Dr. Pranz, if I am correct, 

 never touched the absorption by air at all ; his effects are 

 entirely due to chilling by contact. The mistaking of chilling 

 for absorption causes him to find the same effect in a tube 



Phil. Mag. S. 4. Vol. 23. No. 154. April 1862. T 



