248 HENRY A. BOWLAND 



mechanically, for it is to be noted that those acids which evolve a gas 

 other than hydrogen (perchloric acid, for instance), which is not ab- 

 sorbed by the 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 mag- 

 netic 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 mag- 

 netic 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 flat surface, no matter in how strong a field, it is attacked freely. 

 Whenever 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 diverged, and still further shielding it produced 

 a ridge or point, sharp as if cut with a minute chisel. Mckel and 

 cobalt tend to act like iron, though they are attacked with such diffi- 

 culty that the phenomena are much less strongly 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, 

 first, that the effect previously observed depended not merely on the 

 existence of magnetic force but on its relation to the poles, and, sec- 

 ondly, 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, show- 

 ing 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 



