1912.] 



The Electrical Conductivity of Bacteria. 



333 



that, on the whole, if the effect were determined by transfer across one 

 surface only, a should be higher in strong solutions. But, in the case of a 

 freely suspended body, there will be no orientating couple when the con- 

 ductivities of the liquid and body are the same, for the current flows through 

 both at the same density everywhere. It is then reasonable to conclude that 

 orientation ceases in a solution having the same conductivity as that of the 

 contained bacteria. 



It has been suggested that, since the observations of orientation had been 

 made on thin layers of liquid, it might equally well be explained by 

 electrical endosmose. This is to some extent met by the fact that orienta- 

 tion is quite as marked in deep hanging drops, and in cells several 

 millimetres deep, filled with distilled water. Observations are difficult in 

 these cases on account of continuous thermal streaming in the liquid. It 

 can, however, be readily shown that endosmose is only of importance at low 

 frequencies, and that at frequencies of several thousand a second it entirely 

 disappears. Orientation was first observed in high frequency fields, and is 

 quite different in character from the rigid oscillatory motion caused by 

 endosmose at low frequencies. At 80 periods a second, vigorous movement 

 of all floating matter was observed. Elongated particles instantly moved 

 into line with the flow, and spherical particles, such as carmine, were carried 

 to and fro with an amplitude of about two of their diameters, thus resembling 

 short rods in the field of vision. 



The characteristic feature of the effect was the extreme rapidity with 

 which orientation took place. On substituting an induction coil, with 

 a condenser across the hammer make and break, for the transformer 

 previously used the effect entirely ceased, though the voltage across the 

 drop was now about four times as high. Purified asbestos was then ground 

 to such a degree of fineness that particles were just visible with a No. 4 

 ocular and 1/12 inch objective. These remained in perfect definition when 

 the current from the induction coil was passed through the liquid. Bacteria 

 which were almost invisible in low-frequency fields owing to the rapid 

 oscillatory movement, remained in perfect definition in the coil current and 

 were at the same time actively orientated. 



It may then be said that the mechanical movement of the liquid as 

 a whole under the cover-slip has an amplitude of less than microscopic 

 dimensions at the frequency of the high-tension side of an induction coil, 

 under a gradient of about 1000 volts per centimetre in the film. 



As a further test, a strong emulsion of B. coli communis being under 

 observation with many hundred slightly motile bacteria in view, the voltage 

 from the coil was applied, and all were seen to be orientated. The primary 



2 A 2 



