282 



Prof. W. M. Thornton. 



[June 20, 



glow, is from the above figures 3 x 10 13 charges, each of the magnitude e 

 (4*6 x 10~ 10 E.S.U.), per cubic milhmetre per second. The photograph 

 (fig. 6) shows a faint luminous line proceeding straight outwards from each 

 of the positive points, distinct from the ordinary visible glow. This is only- 

 found after long exposure — three-quarters of an hour — of the photographic 

 plate. It is possible that this is the space in which negative ions are 

 produced and receive inward acceleration in positive point discharge. Some 

 of these would be, no doubt, carried away by the positive wind, and this 

 might account for the presence of negative ions in positive point discharge 

 observed by C. T. E. Wilson and by K E. Campbell. The luminosity may also 

 be from ionisation produced by " Entladungstrahlen."* jSTo such line is to 

 be seen at the negative poles. The length is here 8 mm. At both poles 

 "there is a bright luminous patch on the surface of the metal, but at the 

 positive point there is also some scintillation, possibly due to the negative 

 bombardment, which is not seen at the negative point. The voltage 

 used was not directly measured, but gave sparks 2 to 2 - 5 mm. long between 

 l - 5 cm. diameter brass discharge knobs. On open circuit, that is, with the 

 needle points disconnected, the machine gave T5 cm. sparks, running at the 

 same speed as before. 



4 The second arrangement of discharging points, that used in obtaining 

 figs. 11 to 15 (Plates 9 and 10), differed only from the first in having both 

 positive and negative points brought into the same jar, as in fig. 2. The 

 needle points emerged 2 mm. from the leading-in glass tubes, and were 

 directed to positions on the plates 2\ cm. centre to centre. The stand 

 holding the exposed plate was supported by a sulphur rod, attached to an 

 ebonite base-plate. The leak to earth other than from point to point was 

 exceedingly small. The object of having both points over the same culture 

 was to obtain comparison of exposures in which the current passing was 

 the same for both. The outer part of the agar surface provided a control, 

 which for short exposures may be taken as unexposed, though in certain 

 cases with prolonged exposure the whole plate is cleared. With the two 

 points discharging on to the same surface the action was much stronger 

 than in the first arrangement. 



5. Of the organisms exposed in the first way (fig. 1) photographs of 

 typical exposures are given of B. anthracis, B. pyocyaneus, Sarcina lutea, 

 Pmumococcus ; and in the second (fig. 2) of B. coli communis, B. Friedldnder, 

 3. typhosus, B. asiaticce cholerce, B. dysenterica Shiga. These were all fresh, 

 active growths, though most of them had been often sub-cultured. 



It was found that there was a marked difference between the sensitiveness 



* Vide Sir J. J. Thomson, "Conduction of Electricity through Gases," §§ 242 and 310. 



