tlirough Rarefied Gases. 363 



36. FlnallVj tubes may be prepared in wbicb not only the 

 whole positive light has disappeared, but also the dark space, 

 so that only the negative glow remains, which fills the whole 

 tube. This effect is produced at pressures which are the higher 

 the shorter the tube is. 



The distance between the stratifications is therefore the 

 magnitude which chiefly determines the appearance of the 

 electric discharge. 



37. This magnitude, which was shown to be independent of 

 the length of the tube, varies not only wdth the pressure, but 

 also very much with the dimensions of the tube. I do not 

 give here a very general law which regulates all these pheno- 

 mena, as it "would necessitate a long discussion of experimental 

 details ; the facts may be expressed as follows : — 



If all the cross sections of a tube are altered so that the surface 

 of the tube remains similar to itself, the distance between the 

 stratifications increases. The tubes may have any shape what- 

 ever, either prismatic, conic, cylindric, or ellipsoidal. For 

 example : — . 



If conical tubes are cut out of a given cone at different 

 distances from the vertex, the distance between the strati- 

 fications is the greater the greater the distance from the vertex. 



In cylinders of different widths, the wider they are the 

 greater is the distance between the stratifications. 



The first positive stratification is therefore the more distant 

 from the negative pole the wider the cylinder is. This ex- 

 plains an observation of Wiillner*, who, in his '^Studies on 

 the Discharge of Induction-Currents in Yacuum-Tubes," says, 

 " Increasing the width of a tube diminishes the positive glow." 



38. In conclusion, I may mention the remarkable effect 

 which two surface-elements of two negative poles have on 

 each other, even at a distance of several centimetres. If two 

 pieces of foil, for instance, are used as negative electrodes, the 

 brio'htness of the lio-ht is much increased on the sides of the 

 foil which are turned tow^ards each other ; it is decreased on 

 the other side. 



The strong diminution in size w^hich the second stratification 

 of the negative glow undergoes at the sides which are turned 

 towards each other is also very striking. This fact may be 

 useful when an increase in the brightness of the negative glow 

 is required, as, for instance, for purposes of spectrum-analysis. 

 A short brass tube | centim. wide could be used as negative 

 pole for that purpose. 



* Pogg. Ann. Jubelband. 



