34 



through the air ; place them a little further apart and the spark 

 branches in a zig-zag fashion ; separate the balls a little more, 

 and we get only a spreading brush of blueish light. If this 

 disruptive discharge from metallic balls be examined by the 

 spectroscope, we find 1st, faint lines due to the gas through 

 which the discharge has passed; 2nd, bright lines due to the 

 incandescent vapour of the metallic bodies between which the 

 discharge takes place ; and 3rd, a faint continuous spectrum. 



We learn, therefore, that in the discharge there was a decom- 

 position of the gas separating the electrodes, and a disintegration 

 of the substance of the electrodes. It is believed that the 

 complete molecules of any gaseous medium are incapable of 

 receiving an electrical charge. If a discharge takes place 

 through a quantity of gas it is because of the dissociation of its 

 molecules or of its molecular aggregates. Chemical decom- 

 position is not an accidental attendant, but an essential feature of 

 the discharge, without which it could not occur. 



If we Jessen the density of the gas between the electrodes 

 very interesting features of the discharge are observed. When 

 the density of the air between them is diminished to ^V o^ its 

 normal state, the discharge assumes the form of a band of light. 

 If the air in the tube be diminished to Jj^ the tube is filled with 

 a luminous haze, together with a dark space near the negative 

 ELECTRODE OR KATHODE. Carrymg the exhaustion still further 

 until the air is only y^^ of the original quantity, the discharge 

 proceeding from the positive electrode breaks into bright patches 

 of light separated by dark spaces, but still round the kathode 

 we have Faraday's dark space. Further exhausting the tube we 

 get starting from the anode — the positive electrode : 1st, a 

 stratified column ; 2nd, Faraday's dark space ; 3rd, the negative 

 glow ; 4th, a second dark space round the kathode — this is called 

 Crooke's dark space. Exhausting the tube still further the 

 Crooke's space grows larger, glow and stratification disappear and 

 the kathode becomes the seat of action. 



When the Crooke's dark space occupies the whole bulb 

 remarkable phenomena are observed. The glass of the bulb 

 glows with a characteristic fluorescence. With high exhaustion 

 the particles of gas which remain in the tube have necessarily 

 greater freedom of motion. They do not clash with other atoms 

 and, therefore, impinge with full force on the glass walls of the 

 tube. Hence in the glass vibrations are set up as a 

 consequence of this bombardment which reveal themselves 

 by a beautiful fluorescence. This explanation of Crooke's is 



