MEASURING ELECTRICAL CONDUCTIVITY 35 



tened to thin disks of hard rubber 16 provided with a 

 central opening as shown in Pig. 10, in which A repre- 

 sents the rubber disk (seen in section), B the material, 

 and C another disk of thin rubber or celluloid. These 

 are fastened together by rubber bands, D. For this 

 purpose three projecting knobs are provided as shown 

 in the surface view at the left of Fig. 10. The disk is 

 placed in the frame described under Type A, and the 

 knobs fit in between the glass rods in the manner shown 

 in Fig. 10 (where the rods appear in section). Every 

 other disk is turned upside down so that the knobs of 

 adjacent disks do not touch and interfere with the close 

 packing of the disks. The disks are treated precisely 

 like the disks of Laminaria as described under Type A. 



Most of the experiments on frog skin and on Ulva 

 were made with this type of apparatus. 



Material which cannot be handled in this way may be 

 treated as shown in Fig. 11, where D represents a hard 

 rubber disk with a central opening into which the mate- 

 rial is tightly wedged. The disks are then handled like 

 so many disks of Laminaria. A special type of appa- 

 ratus has been used in experiments on Zostera 17 . 



Experiments were also made with large cells of 

 Nitella, some of which reach a length of 5 or 6 inches 

 and a diameter of a thirty-second of an inch or more. 

 They were packed (Fig. 12) in a trough cut in a block of 

 paraffin (this was then, covered with a plate of glass). The 

 trough was previously filled with a solution: this could 

 readily be changed after the cells were in place. The cur- 

 rent could be sent lengthwise or across the cells : usually 

 both methods were employed. 



M The edges of each piece of tissue are protected by vaseline. 

 17 Cf. Osterhout (191'9, A). 



