26 Prof. Magnus on the Diathermancy 



take place, there can then be no further production of heat, 

 and the heat which was developed at the beginning of the absorp- 

 tion is now gradually given up again. In like manner, when dry- 

 air is blown into the tube, evaporation ceases as soon as all the 

 moisture is removed ; and since there is no further cooling, the 

 pile gradually acquires the temperature of the surrounding 

 medium, even when dry air is forced into the tube in a conti- 

 nuous stream. 



When the current of either kind of air is stopped, the galva- 

 nometer is deflected in the opposite direction, because, supposing 

 moist air to have been used, the water now evaporates again, or 

 if dry air has been blown in, the water is again absorbed. 



It is doubtless scarcely necessary to say that the evaporation 

 and absorption take place more rapidly, and that hence the de- 

 flections of the galvanometer are greater, in proportion as the 

 pressure is increased under which the air is blown into the expe- 

 rimental tube, and also that the deflections are greater when the 

 air is blown into the end of the tube nearest to the thermo-pile. 



If the deflections of the galvanometer are due to the conden- 

 sation and evaporation of watery vapour, they ought to occur 

 when the source of heat is altogether absent. This was found 

 to be really the case, and it was thus proved that absorption of 

 the rays of heat has nothing to do with the phenomena. 



The deflections were, however, much greater when the tube 

 was taken away altogether and the air Was blown directly into 

 the reflector of the thermo-pile, or upon the pile itself. The 

 mirror of the galvanometer mentioned above was moved so far 

 that the scale was no longer visible, and, with the astatic needle 

 galvanometer which I formerly used, the needle was so violently 

 deflected as to be sent quite up to the stop. 



It is unnecessary to say that the air must of course have the 

 same temperature as the pile. 



On removing the lampblack from the face of the pile, perfectly 

 analogous but less violent deflections were obtained. In like 

 manner, similar effects were observed when the pile was coated 

 with substances of different kinds. 



We see by these results how little fitted air is, while in 

 motion, for experiments as to its power of absorption. 



I would gladly pass over the objections which Prof. Tyndall 

 has urged against my method of determining the absorption 

 of radiant heat by gaseous substances*, did I not fear that my 

 silence would be interpreted as an admission that the objections 

 in question are well founded — an admission I am by no means 

 prepared to make. 



In particular, it has been objected to this method, that the 



* Poggendorff 's Annalen, vol. cxii. p. 516. [Phil. Mag. vol. xxii. p. 1.] 



