264 Prof. Tyndall on Recent Researches on Radiant Heat. 



would be quite impossible for me to carry out my experiments 

 with a pile thus circumstanced; for after the instrument had 

 been either heated or chilled dynamically, it required in some 

 cases hours for the needle to return to zero. I may add that I 

 have made these experiments on dynamic heating and chilling 

 with my needles loaded with pieces of paper, so as to render 

 their motion visible to the most distant members of the large 

 audience of the Royal Institution. 



§ 4. In addition to the experiments made with the apparatus 

 which I have described, Prof. Magnus has made two other series 

 with a glass tube one metre in length, and stopped at its ends by 

 plates of glass. His source of heat in this case was a powerful 

 Argand lamp, the rays of which were collected by a parabolic 

 mirror placed behind it. In one series the tube was covered 

 within by a coating of blackened paper, while in the other this 

 coating was removed, the radiation through the tube being in 

 this case augmented by the reflexion from its sides. TVith the 

 blackened tube, Prof. Magnus corroborates the results already 

 obtained for air by Dr. Franz, who makes the absorption of a 

 column of nearly the same length as that employed by Prof. 

 Magnus 3 per cent, of the incident heat. 



The difference between this result and that obtained with the 

 other apparatus of Prof. Magnus, which gave an absorption of 

 11 per cent., might naturally be ascribed to the different kinds 

 of heat employed in the respective cases. But in the series of 

 experiments made with his unblackened tube, and in which the 

 lamp above described was also his source of heat, he finds the 

 absorption of oxygen and of air to be 14" 75 per cent.; and of 

 hydrogen to be 16*23 per cent, of the incident heat. This great 

 difference between the blackened and the unblackened tube, 

 Prof. Magnus ascribes to a change of quality which the heat has 

 undergone by reflexion at the interior surface of the tube, and 

 which has rendered the heat more capable of absorption. I have 

 tried to obtain this result with a glass tube of nearly the same 

 length as that used by Prof. Magnus, but have failed to do so. 

 The absorption of oxygen and air in his tube is 140 times, and 

 the absorption of hydrogen is 160 times what they show them- 

 selves to be in mine. 



Whence these differences ? They are plainly to be referred to 

 a source the same in kind as that which rendered an account of 

 the former ones ; indeed I know not a more instructive example 

 of a single defect running through a long series of experiments 

 faithfully made, and so completely accounting for all the observed 

 anomalies. Prof. Magnus stops his tube with plates of glass 

 4 millimetres in thickness. Now Melloni has shown that 61 

 per cent, of the rays of a Locatelli lamp are absorbed by a plate 



