96 Atomic Theory and Low Voltage Arcs in Ccesium Vapour. 



discussed. This hypothesis suggests that vapours o£ the 

 alkali metals may be so stimulated that the 1st and 2nd 

 subordinate series lines, instead of the principal series, tend 

 to become absorption-lines. 



Experimental. 



The caesium spectrum was photographed for various 

 accelerating voltages, from A 3878 to A 9208, by use of 

 dicyanin-stained plates. The sensitivity of the plates was 

 investigated by density measurements of the spectrum of a 

 black body having a known energy distribution. The general 

 characteristics of the plates were determined and all lines of 

 the csesium spectrum were reduced to an absolute scale 

 of intensity by means of density measurements made with a 

 micro-photometer and consideration of the plate sensitivity. 

 No evidence of group or single-series spectra could be 

 obtained. Thus the ratio of intensities of the first and second 

 lines of the principal series, both of which should appear in a 

 single-series spectrum or above 2*7 volts in a group spectrum, 

 rapidly approaches infinity as the accelerating potential in 

 the arc is decreased. This ratio is 350 in a 7-volt arc, 

 2100 at 4 volts, 10,500 at 3*4 volts, and as near infinity as 

 can be measured at 2'8 volts. Similarly, the intensity ratio 

 of either 1*55 — 2pi or l*5s — 2j> 2 to an y other line approaches 

 infinity at low voltage, proving for the first time the existence 

 of a single-line spectrum rather than a single-series or group 

 spectrum — in the case of csesium the doublet X8521 and 

 X8943. 



The doublet l'os — 2]n is alone produced under excitation 

 of 1*5 to 3*9 volts accelerating field. The intensity of both 

 of these lines gradually increases approximately proportional 

 to the total number of electrons reaching the anode until 

 the ionization potential is reached. At this point a pro- 

 nounced decrease in intensity of these two lines occurs, 

 ;i mounting to the factor one-third. 



This decrease takes place at the voltage at which the com- 

 plete line spectrum is produced, and is readily explainable on 

 the basis of Bohr's theory — in fact, it affords a strong argu- 

 ment for this theory. Thus the lines l'5s — 2p^ are the result 

 of inelastic collision with electrons having velocities between 

 1*45 and 3*9 volts, but as the latter voltage is exceeded, elec- 

 trons, which at a slightly lower velocity would have given 

 rise to l*5s — 2p§, now produce the complete-series spectrum ; 



