Theory and Low Voltage Arcs in Caesium Vapour. 83 



differences, the kinetic energy being in each case equal to 

 the increase in total energy of the atom corresponding to 

 displacements to the 2/>, 3/>, . . . to oo p rings. 



Displacement from 1*55 to mp lt 



,a. Volts. 



2 1-448 



3 2-709 



4 3-184 



5 3-417 



6 3-549 



30 



3-877 



Tlie first and last of these values are readily observable, as 

 above mentioned. That the values corresponding to m = 3, 

 4, etc., have not been observed may be due to the fact that 

 such displacements are very much less probable than the 

 displacement corresponding to m — 2 and the method of 

 detection accordingly insufficiently sensitive. This is sup- 

 ported by the fact that the probability of a transition from 

 the mp ring to the 1*55 ring decreases as m increases — evidence 

 for which lies in the relative intensities of the lines of any 

 series The displacement to the co ring may take place in 

 an infinite number of ways, and hence is readily produced. 

 The value 2*709 is nearly double the value 1'448, and accord- 

 inglv might be mistaken for two successive collisions with 

 different atoms, each resulting in a displacement to the 

 2p\ ring, the probability of which is quite high under ordinary 

 experimental conditions. 



Suppose that the inelastic impact resulted in a displace- 

 ment to the 3jo ring. The electron may return to the 

 l*5s ring directly, resulting in an emission of the line 

 1'5« — 3p, or it might fall to the 3d ring giving the line 

 3d— ojf, then to the 2p ring giving 2j>-3d, and finally 

 to the l'fts ring giving 1*5*— 2p, the total value of all 

 the quanta being equal to the increase in total energy 

 arising from the original displacement to the 3p orbit. 

 Other modes ot: transition from ring to ring are possible. 

 Hence, when a large number of atoms are considered, a cer- 

 tain group of spectral lines may result from a displacement 

 of the type considered. The important point on the basis of 

 Bohr's theory is that for the collision mentioned, lines 

 resulting in displacement from rings beyond 3/' could not 

 exist although the size of the quantum involved might be 



G 2 



