334 



Prof. W. M. Thornton. 



[Jan. 12, 



current was then gradually reduced and the control of the bacteria observed 

 to be weaker, until a point was reached where, though the majority lay in 

 the same direction, at least one-third did not appear to be under control. 

 On raising the voltage all again came into line with the field.] 



2. Method of Measuring the Conductivity of Bacteria. 



From the nature of the case it would be very difficult, if not impossible, to 

 measure the conductivity of bacteria at all accurately by any of the methods 

 at present in use, for example, by passing a current through a fine tube filled 

 with a paste of organisms. Since, however, there is no couple when the 

 conductivities of a rod and the liquid around it are the same, it is only 

 necessary to make trials in a series of liquids of gradually increasing or 

 decreasing conductivity, and to note that in which a rod inclined to the field 

 ceases to show orientation when the current is made or broken. 



To make the measurements of resistance in the present case, a series of 

 12 solutions of sodium chloride in water was made ranging from 0016 to 

 004 grm. of salt per cubic centimetre, of which the corresponding con- 

 ductivities were measured. A drop of the weakest of these was placed upon 

 an ordinary glass microscope slide. To this was added a few bacteria from a 

 pure culture, and a cover-slip dropped over it. The drop was large enough 

 for liquid to exude from the edges of the slip to permit the introduction of 

 platinum wires flat along the edges of the cover-slip in the liquid. The 

 bacteria then being quickly under observation in the microscope, an alternating 

 current was passed and any movement of orientation noted. When they 

 turned into line with the field their conductivity was taken to be higher than 

 that of the liquid. A fresh drop would then be taken from the next higher 

 strength solution, or if the orientation was vigorous from the highest, and the 

 process repeated until that solution was found in which the movement 

 ceased. In the case of high-resistance bacteria a difference of 0*5 mgrm. of 

 salt per cubic centimetre in the solution could be detected in this way. 



In most of the experiments a small laboratory induction coil was used. 

 The voltage between the platinum poles when the current was passing was 

 at least 100, giving a gradient in the liquid of 50 volts per centimetre. This 

 was sufficient to show orientation in distilled water. For demonstration it is 

 more satisfactory to use a quarter inch spark induction coil, the voltage of 

 which falls to a few thousand when connected across the drop. 



The following table gives the results of the measurements on bacteria 

 which are to be found commonly in water or milk. A few others are given 

 of which there were active growths available at the time. The culture 



