Electrification produced by Animal and Vegetable Life. 639 



gradient it is necessary to use a small current, that is to have ti high 

 resistance Ijetween the poles, such as one obtains from a thin film of liquid 

 of high resistivity, 300 to 1000 ohms per centimetre cube. 



The method used was to place a drop of tap water, weak saline or cane 

 sugar solution, on a glass slide, to introduce by a platinum needle the cells 

 to be observed, and to place a |-inch square cover slip over it. The drop 

 was just sufficient to fill the space below the slip without flooding it. 



The electrical pressure was then applied from a hand generator of the 

 kind used in testing the insulation of electric cables, giving about 150 volts 

 when turned slowly, 500 volts quickly, on open circuit. The pressure from 

 a direct-current electric lighting supply with a water resistance or bank of 

 lamps in series also serves well. The current strength should be such that 

 bubbles form at the electrodes very slowly or not at all. The current was 

 led into the drop by means of two fine platinum wires secured to an 

 insulating cross-bar by wax, and attached by flexible wires to a reversing 

 key and the generator. The bar could be raised or lowered freely. The 

 stand holding it was of such a height that the wires were almost horizontal, 

 lying along opposite sides of the cover slip. Using a -ji^-inch oil-immersion 

 lens the cells crossed the field in a few seconds when the generator was 

 turned slowly. 



It is difficult when working with bacteria to avoid the influence of 

 streaming under the slip caused by change of capillarity through electrifica- 

 tion, and it is only by repeated trial that the exact thickness of liquid for 

 dead-beat movement can be obtained. With blood and yeast cells, horwever, 

 the electro-mechanical force on the cell is much greater, and streaming gives 

 less trouble. Its occurrence can in any case be readily seen by the organisms 

 continuing to move after the stoppage of the current. 



Before dealing with living cells it must be stated that almost all finely 

 divided matter can be made to migrate in an electric field in suitable 

 liquids. This has been very fully investigated, notably by Quincke,* Hardy ,t 

 Perrin^ and Burton.§ 



The present results are differentiated from these by the clear division 

 between the movement of the two types of life, in general irrespective of 

 habitat, culture medium or composition of the cell, provided that this is 

 fresh. The intensity of the movement is conditioned largely by the relative 

 conductivities of the organisms and the surrounding liquid. 



* Wiedemann, ' Electricitat,' vol. 2, pp. 1(56 et seq. 



t W. B. Hardy, 'Roy. Soc. Proc.,' B, 1900, vol. 66, p. 110. 



t Jean Perrin, ' Journ. de China. Physique,' 1904, vol. 11, 10, p. 607. 



§ C. V. Burton, 'Phil. Mag.,' April and November, 1906. 



