, through Rarefied Air. 327 



time this density was as low as 2 millims., at another as high as 

 50 millims. In general, when the negative electrode presented 

 a large surface, and the thickness of the air traversed by the cur- 

 rent was considerable, the period during which the intensities 

 increased was of great extent, and that during which they de- 

 creased scarcely appreciable ; on the other hand, when the surface 

 of the negative electrode was very small, the period during which 

 the intensities increased embraced only a small number of de- 

 grees, that during which they decreased being greatly extended. 

 AYhen the electric valve was employed, the intensity of current, 

 corresponding to the most perfect vacuum an ordinary machine 

 is capable of producing, was sensibly equal to zero ; with tubes 

 containing electrodes of metallic wire, the intensity of current 

 corresponding to the most perfect vacuum was always a maximum, 

 but never zero, and in these cases the period during which the 

 intensities diminished was not well defined so long as the distance 

 between the electrodes was small. 



The manner in which Prof. Riess would view these results 

 is already known ; the following explanation is proposed by M. 

 Gaugain : — " lATien an electric current traverses a space con- 

 taining air, the resistance with which it meets ought to vary, as 

 in the case of liquids, with the mean section of the gaseous space 

 which serves to conduct the same. The dimensions of this 

 space cannot, it is true, be immediately ascertained, though it is 

 natural to suppose that they are closely related to those of the 

 sheaf of light which is observable. This hypothesis being ad- 

 mitted, we must conclude that when the negative electrode pre- 

 sents a large surface, the mean section of the current continually 

 increases with the increased rarefaction of the air, for experiment 

 proves that in this case the sheaf of light continually expands. 

 The result is, that the increased section compensates, in a certain 

 measure, for the diminution of conductibility, which is a conse- 

 quence of the diminution of the elastic force of the air. When, 

 on the contrary, the surface of the negative electrode is very 

 limited, the conducting space formed by the reunion of the 

 elementary currents cannot dilate, nothing compensates for the 

 diminished conductibility, and consequently this diminution 

 manifests itself." 



After observing that in the above explanation it is assumed 

 that the same laws which govern the transmission of cuiTcnts 

 through liquids also apply to gases, M. Gaugain cites an expe- 

 riment as an additional justification of the assumption. Au 

 air-pump receiver was procured, in the upper part of which was 

 a copper rod sliding, air-tight, in a leather box ; after exhausting 

 the air under the receiver, the plate of the air-pump was con- 

 nected with one pole of lluhmkorff's apparatus, and the rod with 

 the other ; the usual sheaf of light was seen between the copper 



