50 Prof. Tyndall on the Passage of Radiant Heat 



he employed. One of these was the bringing of the cold gas to be 

 examined into direct contact with his source of heat ; and the 

 other was the bringing of the same gas into direct contact with 

 the face of his thermo-electric pile. In his last paper he urges, 

 in reference to the first point, that my objections do not apply 

 to his apparatus, because in it the column of air is heated at the 

 top. This argument would be strictly valid if the heat could be 

 applied with perfect uniformity to a perfectly horizontal plane, 

 but in practice such perfection is not attainable. The top of 

 Prof. Magnus's recipient is dome-shaped, even where it is in 

 perfect contact with the source of heat, while beyond the limits 

 of this contact, that is to say down the sides of the recipient, it is 

 propagated more or less by conduction. Indeed Prof. Magnus 

 himself states that a portion of the heat effective in his experiments 

 is derived from the glass thus warmed. "The heating of the ther- 

 mometer," he writes, " although due only to conduction and ra- 

 diation, involves a very complicated process. Besides the direct 

 heating through conduction and radiation, reflexion also takes 

 place at the inner surface of the vessel. Further, the portions of the 

 surface adjacent to the vessel of boiling water are heated by conduc- 

 tion, and also radiate heat against the thermometer "*. I have ital- 

 icised the most important part of the passage. Now air in contact 

 with such a surface is substantially in the same condition as in 

 my front chamber, and such air must more or less diminish the 

 temperature of the surface exposed to it. If Professor Magnus 

 fails to detect this, it can, I think, only be that his new appa- 

 ratus lacks the requisite delicacy. Without the actual numbers 

 no safe opinion can be formed upon this point ; the probability 

 is that his total heat is so small that the lowering of the tem- 

 perature of his source by the admission of air into contact with 

 it becomes infinitesimal. 



An important difference between Prof. Magnus and myself 

 consists in the high absorptive power which he ascribes to air. 

 He makes this absorption more than 140 times what I make it. 

 I would here bespeak the reader's attention while I examine the 

 conditions in which Prof. Magnus places his instruments. From 

 his last figure, and also from a passage of his paper, I infer that 

 in his recent experiments the air has free access to the two faces 

 of his pile, the axis of which is vertical. The upper face is 

 furnished with a conical reflector, while the lower one is pro- 

 vided with one of the cylindrical tubes which usually accompany 

 the instrument. Let us reflect for a moment on the processes 

 involved in this arrangement. Professor Magnus keeps the 

 space which contains his pile at a constant temperature of 15° C. 

 Let us first suppose the two faces of his pile to be at the same 

 * Poggendorff's Annalen, vol. cxii. p. 544. 



