Discharge in Rarefied Gases. 383 



exactly in proportion as the excess of the above-mentioned 

 quotient above a whole number changes. 



The changes of colour are also strikingly characteristic of 

 the influence of the kathode, as explained above. The conse- 

 cutive layers of a column of positive light may show very 

 striking differences of colour, even when no differences in 

 form and magnitude can be perceived; these differences are 

 very marked when hydrogen is employed. 



The colour of a layer when the difference of electrodes changes 

 is always, cceteris paribus, dependent on its position with refer- 

 ence to the kathode. Suppose, for example, that we observe a 

 cylinder the positive light of which is divisible into five layers, 

 the one next the kathode being blue, and the following ones in 

 order being yellow, red, greyish red, and grey. Next to the 

 anode we have, therefore, a grey layer. If the distance of the 

 electrodes is now diminished by the length of one layer, whether 

 by the motion of the anode or of the kathode, it is the grey 

 layer which disappears, and we have a greyish-red layer next 

 the anode followed by the rose-coloured, yellow, and blue layers 

 in order. If the distance between the electrodes be further 

 diminished by the length of one layer, the greyish-red layer 

 disappears, and the rose-coloured layer is in contact with the 

 anode, followed by the yellow and blue layers in order. 

 When the poles are caused to approach further so as to leave 

 only two layers, the yellow layer is next the anode, and the 

 blue layer follows it. If, therefore, we count the layers from 

 the anode, then with every change of distance between elec- 

 trodes, the first, second, and every layer change colour; but if 

 we count from the kathode, then the colour of the nth layer is 

 independent of the distance of the electrodes, and each layer 

 present possesses always the same colour. 



Hence the colour of each layer is regulated by the position 

 of the kathode, and depends on its position in the series, 

 counting from the anode. 



Lastly, we may vary the size of the anode indefinitely with- 

 out causing any change in the position of the layers present ; 

 but if the magnitude of the kathode be changed, the position 

 of all the positive layers changes. The smaller the kathode 

 becomes, under conditions otherwise similar, the larger becomes 

 the interval between the kathode and the first positive layer, 

 but the interval between the positive layers is not altered ; so 

 that each single layer lies further from the kathode the smaller 

 the kathode is made. 



We cannot, however, assume that the kathode, or the phy- 

 sical conditions which obtain at the kathode determine the 

 conditions of tension and discharge of the whole stratified 



