C. GENERAL PHYSICS. 



183 



npon liquid in a cistern in connection with a glass thermome- 

 ter or siphon tube up wliich tlie liquid passes, and indicates 

 upon a gauge the heat of the furnace. The defect of this ar- 

 rangement, apparently, lies in the fact that, after being sub- 

 mitted to high temperatures, the rod of copper would take 

 up a permanent elongation, and refuse to expand or contract 

 to the desired extent or with the desired accuracy. 18 A, 

 553. 



CONDUCTION OF HEAT IN GASES. 



The question of the conducting power of gases for heat is 

 one which has received a large share of the attention of emi- 

 nent physicists. The most recent of these researches is that 

 of Winkelmann, who employed for measurement of the heat- 

 conduction the same method which other observers have 

 used ; ^. e., he measured the rate of cooling of a thermometric 

 body within a vessel filled with the gas to be examined. 

 The difiiculty of these experiments lies in the fact that the 

 cooling is caused not only by the conduction of the gas 

 which surrounds the cooling body, but also by the currents 

 which are set up in the gas, and especially by radiation 

 through it. Winkelmann addressed himself particularly to 

 the task of eliminatino- these currents and the radiation. He 

 effected this in one case by varying the pressure of the gas 

 between 760 millimeters and one, since with diminishing 

 pressure the action of the gas currents becomes less. In an- 

 other experiment, he employed various apparatuses in which 

 the cooling body within was always of the same dimensions 

 and the same material, while the outer envelope was varied 

 in size. The value of the radiation was then in all appara- 

 tuses the same, while the conduction varied with the size of 

 the outer vessel, and so furnished data by means of which 

 the radiation could be calculated and eliminated. The re- 

 sults obtained with several of the more common erases are 

 as follows : 



