248 Prof. De la Hive on the Propagation of Electricity 



dark space may, as we have said, be seen to form in front of the 

 negative electrode, and to increase in size in proportion as the 

 pressure diminishes, so as to acquire a length of 10 centims., a 

 length which is independent of that of the gaseous column. 

 Nevertheless, on looking attentively at this dark space, we can 

 discover in it, beyond a perfectly dark and sharply limited slice 

 of 2 to 3 millims. in thickness, a lambent glow of a pale rose- 

 colour, which is visible only in complete darkness. This glow, 

 which has the form of a cone whose base is the last slice of the 

 luminous column, does not appear until the pressure has become 

 very low, much less than that at which the dark space shows 

 itself; it is accompanied by the appearance in the same dark 

 space of some more luminous rings (I have counted as many as 

 four of them), at unequal distances from each other, whose im- 

 mobility and sharply marked outlines contrast with the agitated 

 striae of the rest of the discharge. It should be added that the 

 luminous and stratified part of the discharge, which is by far the 

 longest, is separated from the dark or lambent part more sharply 

 and distinctly the stronger the electric discharge. 



The bluish atmosphere also which surrounds the negative 

 electrode increases in extent considerably as the pressure di- 

 minishes, and nearly at the same rate as the strise. At the same 

 time its brilliance becomes less vivid, and its outer margin less 

 sharply defined. This bluish atmosphere, which at first enve- 

 lopes only the negative knob, at last, in proportion as the pres- 

 sure becomes less, surrounds the whole length of the metallic 

 rod which carries the knob — that is, unless it has been coated 

 with a layer of some insulating substance. This phenomenon 

 indicates, on the part of negative electricity, great facility of 

 flowing out into the surrounding medium when this medium is 

 rarefied. 



The agitation of the strise of the luminous part of the dis- 

 charge becomes very considerable under the small pressure of 2 

 millims. It shows itself at first very vividly near the positive 

 electrode, whence the luminous discharge issues in the form of 

 a cone very much opened out, which as the pressure diminishes 

 becomes more and more cylindrical, until it assumes completely 

 the form of a cylinder with a circular base the centre of which 

 is occupied by the electrode, while at the same time the agitation 

 of the striae becomes general throughout the whole extent of the 

 discharge. 



When the discharge is caused to take place in a cylindrical 

 jar between a metallic ring as positive electrode and a knob oc- 

 cupying the centre of the ring as negative electrode, the bluish 

 atmosphere which surrounds the knob spreads out several centi- 

 metres under a pressure of 2 millims., and its external outline 



