290 MESSRS P. GOTCH AND V. HORSLEY 



about 300 grms. of sulphate of copper crystals were placed, and the whole filled up 

 with distilled water ; a coil of sheet copper, with insulated connections, is immersed 

 in the crystals, and a ring of zinc in the superficial liquid. 



The cell was always prepared forty-eight hours before use, and since the subsequent 

 alterations take place with great regularity and are slowly produced, the cell, 

 when evaporation is prevented, is very steady, and is thus particularly suitable for 

 the purpose indicated. 



The value of a given interval between different points of the compensator wire 

 when the Callaud cell was coupled up with it was always estimated before each 

 experiment and contrasted with that produced by a known fraction of a carefully 

 prepared Daniell cell. This latter was determined by the balancing method employed 

 by mi Bois-REYMOND and described by him in connection with his use of the &quot; Rund 

 Compensator &quot; in his standard work on the technique of physiological experiments 

 involving observations of electromotive changes. The measurements by the com 

 pensator, although made with the Callaud, were thus all translated into terms of 

 a Daniell cell. 



(d.) The non-polarisable electrodes which formed the connection with the tissues 

 consisted in some instances of two pairs. Each pair was then joined to the two 

 screws on one end of a Pohl s reverser (without cross-wires) to the middle screws 

 of which the wires forming the main circuit were connected. Either pair of 

 electrodes could thus be rapidly connected with the recording instrument by 

 turning over the switch. In this way the changes in each of the two prepared 

 sciatic nerves, or in each separated half of the isolated and longitudinally divided 

 cord, could be ascertained one after another in rapid succession. 



In the majority of experiments only one structure was examined, and one pair of 

 electrodes was therefore used and introduced into the main circuit. 



Each electrode consisted of a U-shaped glass tube containing a saturated solution of 

 zinc sulphate with a well amalgamated and annealed zinc dipping into one. limb. 

 This form was preferred in consideration of the experimental necessity of keeping the 

 electrodes exposed for an hour or more, in order to minimize the alterations due to 

 evaporation. 



It was of the first importance that the connection of the electrode with the 

 investigated structure should be so firmly attached as to suffer no displacement from 

 any chance movement of the animal, and that they should be of such a nature as to 

 readily permit an adequate isolation of the structure from all its neighbouring tissues. 

 This was effected in the case of each electrode by means of thread cables made out of 

 the soft strands of such wicks as are used in ordinary paraffin lamps, since these 

 readily imbibe and become soaked with moisture. Two strands 10 centims. long, were 

 usually taken and soaked in 6 per cent, solution of NaCl ; they were then plastered 

 with powdered kaolin, made into a paste in similar liquid, and thus united into a 

 double cable. This cable was passed under the structure investigated and tied gently 



