46 Prof. Tyndall on the Passage of Radiant Heat 



being produced by the sudden reabsorption of the atmospheric 

 vapour when the dry air is intercepted. Air artificially moist- 

 ened produces still larger deflections than the above. 



Such effects were well known and duly guarded against. 

 Indeed, it is to me interesting to notice my own experience in 

 this inquiry reproduced years subsequently in the experience of 

 Professor Magnus. I never had the least doubt of the correct- 

 ness of his results ; but, for the most part, they have absolutely 

 nothing to do with mine. We are equally successful in our 

 efforts. His object, for example, is to bring the hygroscopic cha- 

 racter of rock-salt into strong relief, and he succeeds in wetting 

 the plates ; my object is to avoid this source of disturbance, and 

 I am equally successful. He, by blowing vigorously into his 

 tube, urges the air against the face of his pile, and obtains the 

 effects due to condensation and evaporation ; I, by operating cau- 

 tiously and permitting the air to enter the tube so slowly and at 

 such distances from the source and from the pile that neither of 

 them is affected by it, obtain the effects due to absorption. One 

 great feature of the case, however, is, that while the results of 

 Professor Magnus have been known to me for years, and while 

 I can produce them on a large scale at any moment, he has not 

 yet succeeded in reproducing mine. " Never," he writes, " in a 

 single instance has the deflection indicated a greater absorption 

 by the humid air." 



As soon as I had read the last paper of Professor Magnus, I 

 felt that it would be useless on my part to reiterate what I had 

 already so often affirmed, and I therefore wished to subject my 

 experiments to the scrutiny of an independent observer. Mr. 

 Faraday had already seen those experiments, and it is purely *my 

 reluctance to give him trouble that prevented me from asking 

 him to witness them again. Next to him I could hardly find a 

 man whose testimony on such a subject will have greater weight 

 than that of my colleague, Dr. Frankland ; and he, at my request, 

 kindly undertook to satisfy himself upon the points at issue. 

 I mounted the apparatus, and left it entirely in his hands, 

 and he has favoured me with the following account of his 

 observations : — 



" My dear Tyndall, — At your request I have made a num- 

 ber of experiments on the comparative transcalency of common 

 air, and of air deprived of its moisture by contact with mo- 

 nohydrated sulphuric acid. The apparatus which I used was 

 that described by you in the Philosophical Transactions for 

 December 1862*. It was exclusively under my own control; 

 and I arranged the details of manipulation in such a manner as 



* [And also in the present Number of the Philosophical Magazine. — J. T.] 



