Action of a Magnet on Chemical Action. 45 



water, tend to produce little magnetic effect compared with 

 those which act without the evolution of any gas. 



As to the actual cause of the protective action exercised by 

 the magnetic field, all these experiments go to show that it is 

 quite independent of the substance acting, with the exception 

 above noted, and is probably due to the attractive action of the 

 magnet on the magnetic metals forming the poles subjected to 

 chemical action, as we have before explained. 



In the first place, whenever iron is acted upon chemically in 

 a magnetic field those portions of it about which the magnetic 

 force varies most rapidly are very noticeably protected, and 

 this protection as nearly as can be judged varies very nearly 

 with the above quantity. Wherever there is a point there is 

 almost complete protection, and wherever there is a fiat surface, 

 no matter in how strong a field, it is attacked freely. When- 

 ever in the course of the action there is a point formed, the 

 above condition is satisfied and protection at once appears. 

 Thus, in ' the steel bars experimented on, whenever the acid 

 reached a spot slightly harder than the surrounding portions it 

 produced a little elevation from which the lines of force di- 

 verged, and still further shielding it produced a ridge or point, 

 sharp as if cut with a minute chisel. ]S r ickel and cobalt tend 

 to act like iron, though they are attacked with such difficulty 

 that the phenomena are much less strongly marked. With the 

 non-magnetic metals they are completely absent. Now, turn- 

 ing to the experiments with the wires connected with a galvan- 

 ometer, 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, first, that the effect previously ob- 

 served 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, 

 while 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 excep- 

 tion. It therefore appears that the particles of magnetic mate- 



