204 



Magnetic Qualities of Nickel. 



[May 17, 



other modes of stimulation, have the same seat, and that the opposi- 

 tion between them is in accordance with a principle applicable in 

 common to the excitable structures of plants and animals, viz., that 

 the property which renders a structure capable of undergoing excitatory 

 change is expressed by relative positivity, the condition of discharge by 

 relative negativity. 



With reference to the mode of action of the voltaic current, the 

 effect produced in the unexcited leaf is compared with that observed 

 in the unexcited electric organ of the -skate or the torpedo, in both of 

 which, as in the leaf, it is observed that, although the after-effect of a 

 current led across the disks or plates is to increase the difference of 

 potential between its two surfaces, whichever way the current is 

 directed, the effect is much greater when the direction of the external 

 current coincides with that of the normal electromotive action of the 

 organ than in the opposite case. 



It is further shown that the electromotive changes concerned in 

 " modification " and " excitation " have their seat at the upper surface 

 of the lamina. If, as the author believes, all these changes depend on 

 difference of physiological activity between adjacent excitable cells or 

 strata of cells of which the protoplasmic linings are in continuity, it 

 must be supposed that when the leaf is at its prime, the most super- 

 ficial strata are positive to those subjacent, and that as the former 

 lose their pristine susceptibility of excitatory change, the physio- 

 logical, and consequently the electrical, difference between them is 

 diminished, annulled, or reversed. 



The fourth section of the paper is devoted to an investigation made 

 in 1887, of the events of the first second after excitation made with 

 the aid of a pendulum-rheotome specially adapted for the purpose. 

 The fifth contains the description of the records obtained by photo- 

 graphing the electric phenomena of the excitatory reaction, as ob- 

 served with the aid of the capillary electrometer, on rapidly moving 

 plates. Both of these series of observations serve to confirm and 

 complete the results obtained by other methods. 



II. "Magnetic Qualities df Nickel." By J. A. EwiNG, F.R.S., 

 Professor of Engineering, University College, Dundee, and 

 G. C. Cowan. Received April 26, 1888. 



(Abstract.) 



The experiments described in the paper were made with the view 

 of extending to nickel the same lines of enquiry as had been pursued 

 by one of the authors in regard to iron (' Phil. Trans.,' 1885, p. 523). 

 Cyclic processes of magnetisation were studied, in which a magnetising 



