1912.] 



The Electrical Conductivity of Bacteria. 



337 



is reached. The ratio of the specific resistance of the medium to that of the 

 organism in broth, taking a mean value of 87 ohms per centimetre cube for 

 the latter, is 87/35, that is 2 - 5. Taking the value for blood, which is also 

 Waller's value for muscle, as that for the host as a whole, the ratio is 200/90, 

 that is 2-2. The numerical coincidence is probably accidental, but it suggests 

 that there may be, within limits, an adaptation of the organism to its 

 surroundings, such that its electrical conductivity is always the greater. 



4. The Influence of Unidirectional Currents. 



On W ater. — In the experiments about to be described all possible care was 

 taken to control the temperature rise consequent upon the passage of the 

 current. With the exception of those on the influence of alternating 

 current on milk, the temperature of the liquid did not rise above 30° C. In 

 the case of water a long and narrow form of sterilising cell was used, to keep 

 the cell cool and at the same time to avoid the influence of any gas liberated 

 at the electrodes when using direct current. Two No. 1 microscope cover- 

 slips, each 2 - 5 cm. wide and 9 cm. long, were cemented together with paraffin 

 wax to form a V trough 0'15 cm. wide, open at the top. Narrow strips of 

 platinum foil were inserted at each end, the greatest distance between them 

 being 4 - 5 cm. Even with the highest unidirectional pressures used, i.e. 

 480 volts, across the cell, and a current of 0"5 ampere per square centimetre 

 in it, the fine bubbles of gas which escaped slowly from the electrodes could 

 not be traced more than 0*3 cm. on the surface towards the centre of the 

 cell. The stream of cooling water was on. a level with that inside. The 

 resistance between the poles with tap water in the cell was about 

 50,000 ohms. Quantities of |- c.c. of an emulsion of B. coli communis in 

 non-sterilised tap water were introduced into the trough and exposed to a 

 voltage gradient of 130 volts per centimetre for various intervals of time. 

 With young fresh cultures an exposure of one minute caused slight but 

 perceptible diminution in the number of colonies on a Petri dish culture 

 compared with a control plate. 



An exposure to direct current for five hours at a voltage gradient of 

 210 per centimetre and a current density of - 3 ampere per square centi- 

 metre reduced the colonies in a ratio approaching 100 to 1. After 24 hours' 

 exposure one colony developed, the control plate having a growth too dense 

 to be counted with certainty. 



The most efficient form of cell for use with direct current is one in which 

 oxygen liberated at the positive pole bubbles through the liquid in escaping, 

 lor this purpose a cell with a short distance between pole plates placed in 

 a horizontal plane in the liquid is convenient, taking a large current at low 



