56 M. W. Feddersen on Thermo diffusion of Gases. 



there appeared a slow displacement of the air column in a di- 

 rection through the plug, from the cold to the heated end, 

 sometimes quicker, sometimes slower, sometimes stronger on 

 one side and sometimes on the other. 



Spongy platinum was the first substance with which I experi- 

 mented, in the expectation that, in consequence of its great ab- 

 sorptive capacity, especially for oxygen, with it rather than any 

 other substance an action would be perceptible. Into a tube of 

 about 3J millims. diameter, a length of about 60 millims. of 

 spongy platinum was pressed, which had been heated to redness 

 two days previously. When I inserted a drop of mercury in the 

 accessory tube, applied preliminarily to the heated side only, 

 there was indeed a displacement backwards of the drop, but to 

 a considerable degree with strong heating only. In presence 

 of the minuteness of the pressure -force set free, the drop of 

 mercury was evidently too heavy. I substituted for it sulphuric 

 hydrate ; and immediately, with the difference of temperatures 

 10° (that of the room) and 100°, the expected movement began 

 in a striking manner. As an example, out of various experi- 

 ments I will only mention that, in the eight hours period of 

 observation, the drop once moved 195 millims. away from the 

 heated end. Within this entire interval, however, the speed of 

 the motion varied. In another instance, the platinum being 

 fresh from incandescence, a brisker movement took place. 

 Nevertheless the displacement of the drop was always slow ; so 

 that changes in the temperature of the apartment (of course not 

 heated) may have exerted a sensible influence. 



I therefore took a shorter tube about 12J millims. in diameter, 

 inserted therein a plug of recently incandescent spongy plati- 

 num of 31 millims. length, and placed at each end a long tube 

 of glass 3^ millims. in diameter, stopped by a drop of sulphuric 

 acid. One end of the plug was exposed to a temperature of 

 about 200°*, while the other end was left to cool freely down 

 to the temperature (8°) of the unwarmed room. On account of 

 the great heat-conducting capacity of the platinum, this end was 

 also not inconsiderably heated. The following were the ascer- 

 tained displacements in the accessory tubes, all in the direction 

 from the cool, through the plug, to the hotter side : — 



* Compare what is said further on, under " Gypsum." 



