112 Messrs. Rowland and Bell on the 



marked. With the non-magnetic metals they are completely 

 absent. Now, turning to the experiments with the wires 

 connected with a galvanometer, the same facts appear in a 

 slightly different form. 



When the poles were placed perpendicular to the lines of 

 force instead of parallel to them, the magnet produced no effect 

 whatever ; showing, firstly, that the effect previously observed 

 depended not merely on the existence of magnetic force, but 

 on its relation to the poles, and, secondly, that when the poles 

 were so placed as to produce little deflection of the lines of 

 force, the protective effect disappeared. 



When the pointed pole was blunted the effect practically 

 disappeared, the poles remaining parallel to the lines of force, 

 and when plates were substituted for the wires no effect was 

 produced in any position, showing that the phenomena were 

 not due to the directions of magnetization, but to the nature 

 of the field at the exposed points. In short, whatever the 

 shape or arrangement of the exposed surfaces, if at any point 

 or points the rate of variation of the square of the magnetic 

 force is greater than elsewhere, such points will be protected, 

 w r hile if the force is sensibly constant over the surfaces ex- 

 posed, there will be no protection at any point. With all the 

 forms of experimentation tried this law held without exception. 

 It therefore appears that the particles of magnetic material on 

 which the chemical action could take place are governed by 

 the general law of magnetic attraction and are held in place 

 against chemical energy precisely as they would be held 

 against purely mechanical force. To sum up : — 



When the magnetic metals are exposed to chemical action 

 in a magnetic field, such action is decreased or arrested at any 

 points where the rate of variation of the square of the mag- 

 netic force tends towards a maximum. 



It is quite clear that the above law expresses the facts thus 

 far obtained, and while in any given case the action of the 

 magnet is often complicated by subsidiary effects due to cur- 

 rents or bye-products, the mechanical laws of motion of 

 particles in a magnetic field hold here as elsewhere, and cause 

 the chemical action to be confined to those points where ihe 

 magnetic force is comparatively uniform. 



The effect of currents set up in the liquid during the action 

 of the magnet cannot be disregarded, especially in such expe- 

 riments as those of Nichols (' American Journal of Science,' 

 vol. iii. pp. 131, 272), where the material acted on was powdered 

 iron and the disturbances produced by the magnet would be 

 particularly potent. The recent experiments of Colardeau 

 (Journal de Physique, March 1887), while perhaps neglecting 



