22 



Prof. Magnus on the Diathermancy 



Prof. Tyndall obtained for the absorption of radiant heat and 

 those which I found myself. It is true that both of us, inde- 

 pendently of each other, and following perfectly distinct methods, 

 obtained for nearly all gases values which are as concordant as 

 can be expected from measurements of this kind, although I 

 obtained a greater value than Prof Tyndall for the absorption by 

 dry air compared with a vacuum. Between our observations for 

 air saturated with moisture at the common temperature there is, 

 however, a great disagreement. For while I have observed only 

 a very small difference between the power of transmitting heat 

 possessed by dry air and that possessed by moist air, Prof. Tyn- 

 dall* finds the absorption by moist air so great that, the absorp- 

 tion of dry air being taken as unity, that by the undried air of 

 the laboratory was — 



On the 23rd of October, 1861 .... =63 



„ 24th „ .... =62 



„ 29th „ .... =65 



„ 31st „ .... =56 



„ 1st of November .... =50 



„ 4th „ .... =58 



„ 8th „ .... =49 



„ 12th „ .... =62 



Prof. Tyndall employs in his method two sources of heat which 

 are placed opposite the two faces of the thermo-pile furnished 

 with its conical reflectors, and they are adjusted until both faces 

 of the pile receive an equal amount of heat, and the galvano- 

 meter is consequently not deflected. For this purpose there is 

 placed in front of one of the sources of heat, which we may call 

 the compensator, and which consists of a cube filled with boil- 

 ing water, a screen made of two parallel tin plates, by moving 

 which the quantity of heat which falls upon one face of the pile 

 can be so regulated as to be always equal to that which reaches 

 the opposite face. 



Between the thermo-pile and the principal source of heat, 

 which consists of a plate of copper heated to about 300° C. by a 

 gas-flame, is placed the experimental tube, which can be closed 

 at both ends with plates of rock-salt, and can then be employed 

 either vacuous or filled with the gases or vapours to be examined. 

 It has, however, also been used by Prof. Tyndall for comparing 

 the absorption by dry and moist air without using the closing 

 plates, the two kinds of air having been allowed to flow into the 

 tube at one end, while the other end was connected with an air- 

 pump. By this latter method Prof. Tyndall obtained a deflec- 

 tion of 30° of his galvanometer when he allowed moist air to flow 



* Phil. Trans. 1862, p. 89. .[Phil. Mag. vol. xxiv. p. 426.] 



