86 DK. S. R. MILNER ON THE NATURE OF 



discharge is nearly as powerful as that of the cathode. It is worthy of note that, in spite of this, in the 

 line 4481 there is a distinct separation between the anode and cathode streamers towards the end of the 

 discharge. This shows that the streamer corresponding to this line is only produced when the current 

 and electric force exceed a certain minimum value. The same effects are to be seen in the spectra of 

 calcium, lead, and mercury. 



Mercury. 



Fig. 27. Capacity 0-0106 microfarad, no inductance, velocity 670 metres per second. 

 Fig. 28. Capacity 0-0106 microfarad, inductance 40 microhenries, period 4'1 micro-seconds, velocity 

 890 metres per second. 



Fig. 29. Capacity 0-0223 microfarad, inductance 170 microhenries, period 12'2 micro-seconds. 

 The sparks were taken from poles of amalgamated zinc, and show the lines of both metals, as follows : 



Arc lines. Duration 58 micro-seconds in fig. 27, diffuse streamers in fig. 28. 

 (5790, 5769), 5461, 4359, 4047, 3650 due to mercury. 

 4809, 4721 due to zinc, but behaving quite similarly to the mercury as regards duration and 



velocity. 

 Spark lines. 



3984 (mercury). The streamers differ very little in sharpness from those of the arc lines in 



fig. 28. In fig. 29 they are sharper and considerably diminished in intensity. 

 (4924, 4911) (zinc). Sharp streamers in fig. 28, which disappear with the increased inductance 

 in fig. 29. 



Zinc. 



Fig. 30. Capacity 0'0171 microfarad, inductance 40 microhenries, period 4' 8 micro-seconds. 



The arc lines 6363 ;it the top of the figure (very faint), and 4809, 4721, 4609, in the centre in both 

 cases form blurred bands overlapping the sharp streamers of the spark lines (6102, 6022) and (4924, 

 4911) which show through them. 



Sodium. 



Fig. 31. Capacity 0'0223 microfarad, no inductance, velocity 130 metres per second. 



Fig 32. Capacity 0'0223 microfarad, inductance 95 microhenries, period 9'1 micro-seconds, velocity 

 440 metres per second. 



The only line clearly visible is the D line, which has a very great duration (170 micro- seconds in fig, 31). 

 In spite of this it shows moderately well-defined streamers in fig. 32. There is a striking difference 

 between the air-line spectrum in the two sparks in this photograph, the right-hand one showing a number 

 of fine lines which are not air lines, and which are possibly due to the spark having occurred through 

 sodium vapour. 



Tin. 



Fig. 33. Capacity 0'0171 microfarad, inductance 40 microhenries, period 5 - micro-seconds, velocity 

 800 metres per second. 



The two blurred bands which last in the centre of the spark gap for some time after the oscillations 

 have ceased are the two strong arc lines 4525 and 3801. The streamers apparent in the earlier parts of 

 their courses are probably due to the spark lines 4586 and 3861, and not to the arc lines themselves. The 

 spark lines, like those of bismuth, although not to the same extent, vary considerably in the number of 

 repetitions which their streamers undergo. The strongest of the condensed spark lines, which shows very 

 brightly and as of very short duration in the inductionless spark, forms the bright but very rapidly fading 

 streamers just below the lower arc line (3745 probably). The well-defined streamers at the top of the 

 figure are those of the lines 5801, (5589, 5563), 5333. 



