510 Prof. Maguus on the Conduction of Heat by Gases. 



of the absorbent, the mercury 

 rises in the cup to supply the 

 place of the water thus absorb- 

 ed, and the column CD de- 

 scends. This goes on until the 

 force of absorption is balanced 

 by the downward pressure of 

 the mercury in the tube LE, 

 that is, by a column of mercury 

 measured by the descent of the 

 mercury in the tube C D. The 

 following results were obtained 

 by means of this apparatus : — 



Unsized paper laid upon the plate caused the column of mer- 

 cury C D to descend 2'3 inches, which is equivalent to a column 

 of 31 inches of water. 



Brick caused the column of mercury CD to descend 13 

 inches, which is equivalent to a column of 17 inches of water. 



Plaster of parts, encased in a glass tube, caused the mercury to 

 descend 2*7 inches, which is equivalent to a column of 36 inches 

 of water. And so on to other absorbents. 



Hastings, November 10, 1860. 



[To be continued.] 



LXVIII. On the Conduction of Heat by Gases. By G. Magnus*. 



THE cooling of a body in vacuo depends simply on the ex- 

 change of heat by radiation between the cooling mass and 

 the encircling envelope. If the space contains gas, an ascending 

 current is formed, which accelerates the cooling, added to which 

 the property which the gas has of transmitting heat, or its 

 diathermancy, concurs in producing cooling, provided the gases 

 can conduct heat. Dulong and Petit, in enunciating their law 

 of the loss of heat, have neglected the last two actions, manifestly 

 because they are infinitely small compared with the influence of 

 the ascending currents. Since then it has been universally ad- 

 mitted that the differences in the cooling of different gases depend 

 on the different mobility of their particles. Cooling takes place 

 much more rapidly in hydrogen than in other gases. With the 

 same amount of heat this gas expands not more, but less than 

 atmospheric air ; the changes in density in the former gas are 

 less than in the latter. But it is the difference of specific gravity 

 which produces currents. If, therefore, different gases by contact 

 with a warmer body all become equally heated, the currents in 



* Translated from the Bericht der Berliner Akademie, 1 860, p. 485. 



