and Diathermancy of Air and Hydrogen. 415 



source of beat, I have not yet been able experimentally to de- 

 termine. Glass cylinders of larger diameter would be neces- 

 sary in order to have the cooling-water more distant from 

 the junction of the thermopile. These experiments, though 

 unsuccessful, have, I think, shown why the results obtained by 

 other observers, whose names alone are sufficient guarantees 

 for the correctness of their statements, differ so very widely 

 from those obtained by me. 



According to the first observations made by Magnus, dry air 

 absorbs 18 per cent, of the heat-rays proceeding from a dark 

 source*. But it has already been pointed out that sources of 

 error sufficient to account for such differences were not taken 

 into consideration in his method. His thermometer, parti- 

 cularly, was not sufficiently protected from currents of warm 

 air. 



Magnus himself does not seem to have placed much confi- 

 dence in those experiments, as far as they had reference to 

 the diathermancy of different gases ; for in a second series of 

 experiments t, undertaken with the view of investigating the 

 diathe]*mancy of gases, his apparatus was essentially altered : 

 for the mercury thermometer, as described in his memoir, 

 a thermoelectric pile was substituted. This was placed in 

 a glass cylinder 175 millims. in height by 100 millims. in 

 width, and surrounded by water of constant temperature. 

 The source of heat was at least 200 millims. above the face of 

 the pile ; and the rays before reaching the face had to pass 

 through a narrow channel of only 24 millims. width. With 

 this apparatus Magnus found that the air absorbed only 11 

 per cent, of the incident rays. 



Here there could be no possibility of a disturbance from air- 

 currents ; but other difficulties presented themselves. The 

 thermopile, whose bars were only 30 millims. long, was wholly 

 within the vessel for the reception of the gases ; one face was 

 turned towards the source of heat ; the other was fixed to the 

 plate of an air-pump by means of a cork, into which the lower 

 end of the cylindrical casing penetrated for a short distance. 

 This arrangement prevented, or at least impeded, a free circu- 

 lation of the gases around the lower face of the pile. 



It is probable that, as the experiment lasted several minutes, 

 the heat received by the blackened front face of the pile was 

 soon propagated to the lower face (where, as already stated, 

 the elimination of heat was greatly impeded), and very soon 

 raised its temperature above that of the surrounding medium. 



In consequence of this, the difference in temperature of the 



♦ Phil. Mag. vol. xxii. p. 10. f Ibid. p. 95. 



