324 ELECTRO-PHYSIOLOGY. 



to the positive wire and form sulphate of zinc, which is immediately set free in 

 the liquid, and keeps it saturated as at first ; at the negative pole zinc is pre- 

 cipitated, which, finding an excess of mercury, becomes amalgamated, and leaves 

 the wire in nearly the same state as before. There is, therefore, no alteration 

 produced by the passage of the current either in the liquid or the electrodes, and 

 this explains the absence of the secondary polarity. 



Hence the practice is now generally adopted, in all electro-physiological ex- 

 periments made with the galvanometer, of having two strips of amalgamated 

 zinc, soldered to the extremities of the copper wire and immersed in a saturated 

 Boluliou of sulphate of neutral zinc which fills the two cups furnished with the 

 cushions above described. 1 have recently introduced a useful modification, 

 as follov\'S : instead of strips of zinc I use an amalgam of zinc which half fills a 

 glass tube bent in the shape of U, but of very different diameter in the two 

 branches. Into the narrow tube filled with the amalgam I introduce the copper 

 wires of the galvanometer ; on the amalgam which is in the wider branch I pour 

 a saturated solution of sulphate of zinc, so as to fill this branch almost entirely ; 

 each of these branches has a wide rostrum or beak, flattened like a duck's bill, 

 in which the liquid spreads itself in a thin stratum and fulfils the otfico of the 

 cushions. With this modification the metallic appendages continued to be 

 homogeneous without renewing the amalgamation, and we avoid the soiling of 

 the cushion with the liquids which moisten the pieces of muscle, an inconvenience 

 which obliges us to wash frequently the actual cushions in the solution of the 

 sulphate. Au experiment will evince that by these means we avoid the secondary 

 polarity. I place any small body, a piece of glass for instance, against the needle 

 of the galvanometer to prevent any deviation occasioned by the current of a frog 

 or battery of frogs. This obstacle does not hinder the needle from deviating in 

 the opposite direction — in that, namely, in which it would be made to deviate by 

 the secondary polarity on closing the circuit between the two cups alone, imme- 

 diately after the removal of the frogs. I cause, in effect, the current of a battery 

 of six or eight frogs to pass and notice what occurs, on seeing the needle ap- 

 proach and press against the obstacle. After some seconds I remove the muscu- 

 lar battery and immediately afterwards bring the cups into contact, and I obtain 

 no sign of deviation. I conclude, then, from this experiment that the method I 

 adopt renders these experiments of electro-physiology independent of the sec- 

 ondary polarity. Moreover, I propose to show that by causing the current of a 

 frog to pass with these electrodes of zinc, the needle is promptly fixed, and 

 indicates a deviation much greater than that which would be obtained by using 

 electrodes of platina in water either distilled or slightly saline. 



In terminating this exposition of the experimental method, I will state finally, 

 that, for a comparison of the electro-motor power of two pieces of muscle or 

 other animal tissue, I have always used with invariable success the principle of 

 opposition which I have already brought to your notice. For this purpose the 

 two electro-motors are placed on the customary surface of gutta-percha in com- 

 munication with one another and in such a way that their currents arc in an 

 opposite direction to one another. On now touching the extremities of this 

 double battery with the two cushions of the galvanometer, we shall have, if 

 there be a difi'erence of electro-motor power, a differential current in the direction 

 of the stronger electro-motor, which will be independent of the internal resist- 

 ance. AVould we next measure in some manner the electro-motor power of the 

 different muscular elements, we use, as was first done by Giulio Reguault, an 

 eJcctro-motii'c unit, which may be a thermo-electric pair, bismuth and copper, 

 in which the union is maintained at a constant difference of temperature, being 

 the one at O'^ and the other at 100''. The electro-motor power of a muscle or 

 other animal tissue may be expressed by the number of the electro-motive units 

 required in order that there shall be no difi'erential current. 



