Page:Proceedings of the Royal Society of London Vol 69.djvu/202

188 The above experiment shows that the fallacy of the electrodes may be of complicated origin, and that care should be taken to ascertain, by separately testing the electrodes, that it has been eliminated. The inter- pretation of the fallacy that has been given above is purely tentative, and whether right or wrong does not form part of the principal argu- ment, which is based upon experiments in which the fallacy has been eliminated. Nevertheless the fact that it might be present is important to bear in mind, especially in cases where reactions of low voltage (below O'OOl volt) are under examination. But, as previously stated, the fallacy, even if present, would not interfere with conclusions based on reactions of high voltage (above O'Ol volt).

The fallac}^ of the electrodes is easily recognised and easily avoided. To recognise the fallacy it is sufficient to bring the clay pads of the two electrodes into contact, exactly compensating any accidental current, and then pass testing currents through the circuit, just as when a test object is interpolated between the electrodes. There should be no movement of the galvanometer spot, or at most the slight movement due to defect of compensation or trace of polarisation, in either case less than the movement produced by O'OOOl volt thrown into circuit.

To remove the fallacy, if recognised, it is usually sufficient to care- fully reamalgaraate the xinc rods. From preliminary experiments with xinc rods, unamalgamated and amalgamated, in water and in saturated solution of ZnSO 4, it appeared that fallacy of electrodes might lie due to a defectively amalgamated kathodic xinc surface rendered electronegative by an exciting current.

The regular electrical effect of indirect excitation is an ingoing current.

The usual and principal electrical effect of direct excitation is an outgoing current.

An ingoing current may be obtained by direct excitation imme- diately after death.

I think it probable that both ingoing and outgoing forces may co-exist in the excited skin at the same moment, and that the galvano- metric deflection is an expression of this resultant.

This co-existence of two opposite forces is, however, less clearly evident in the case of the cat's skin than in those of the frog's skin and of the frog's eyeball.