in highly Rarefied Elastic Fluids, 257 



comparable with each other, since it is the same jet of electricity 

 which traverses successively these different unequally conducting 

 parts. 



If the platinum disks are placed in a part of the jet which is 

 at one-third of the whole distance from one of the electrodes, and 

 consequently at two-thirds from the other, under a pressure of 2 

 millims. the derived current gives a deflection of 8°, in the case of 

 air or nitrogen, when the electrode nearest to the disks is nega- 

 tive, and 12° when it is positive. In hydrogen the corresponding 

 deflections are 20° and 36°. Hence the conductivity of the ga- 

 seous column diminishes gradually from the dark space, where 

 it is a maximum, to the space near the positive electrode, where it 

 reaches a minimum. 



The disks being always in the same part of the jet, the inten- 

 sity of the derived current may serve as a tolerably accurate ex- 

 pression for the relative resistances of the different gases at dif- 

 ferent degrees of pressure, provided that care is taken to keep the 

 total intensity of the principal current the same in each case by 

 means of a rheostat. This is an investigation which I have in 

 hand, and which is not yet finished. 



We see therefore that the dark space near the negative elec- 

 trode offers much less resistance to the passage of the electric 

 current than is offered by the luminous part near the positive 

 electrode. Hence it follows, for the same reason that the por- 

 tion of the gas whose conductivity is lowest is more luminous 

 than that which has the highest conductivity and which remains 

 almost dark, that the temperature of the former must be higher 

 than that of the latter. This has been fully borne out by ex- 

 periment. 



Two mercurial thermometers with cylindrical bulbs were placed 

 inside a tube 16 centims. long and 4 in diameter, each at a dis- 

 tance of 1 centim. from one of the electrodes, at which distance 

 it was found that the heating or cooling effect of the electrodes 

 was imperceptible. Any effect which they might have produced 

 would have been cooling, as was proved by bringing them nearer 

 to the bulbs of the thermometers ; considering their dimensions 

 (solid metallic knobs, of a centimetre in diameter) , this is not to 

 be wondered at. 



It did not take long, when the electric discharge was made to 

 pass through rarefied nitrogen or hydrogen, to perceive that the 

 temperature indicated by the thermometer in the dark space near 

 the negative electrode was very different from that shown by the 

 thermometer in the luminous space near the positive electrode. 

 These differences maintain nearly a constant ratio between the 

 pressures of 1 and 10 millims., even although the absolute tem- 

 peratures, with which they must not be confounded, vary with 



Phil Mag. S. 4. Vol. 33. No. 223. April 1867. S 



