Action of a Magnet on Chemical Action. Ill 



throw of the galvanometer for various small known values of 

 the E.M.F. The values found varied greatly, ranging from 

 less than 0*0001 volt in case of the acids evolving hydrogen, 

 up to 0*02 or 0*03 volt with nitric acid and certain salts. 

 These were the changes produced by the magnet, while the 

 initial electromotive forces normally existing between the 

 poles would be, roughly speaking, from 0'001 to nearly 0*005 

 volts, never disappearing and rarely reaching the latter figure. 



From these experiments it therefore appears that the protec- 

 tive action of the magnetic field is general, extending to all 

 substances which act chemically on the magnetic metals. 

 While this is so, the strongest effect is obtained with those 

 substances which act without the evolution of hydrogen. But 

 the series is really quite continuous, perchloric acid for 

 instance producing but little more effect than hydrobromic, 

 while this in turn differs less from perchloric than from an 

 acid like acetic. It seems probable that the action of the 

 hydrogen evolved is partially to shield the pole at which it is 

 evolved, and lessen the difference between the poles produced 

 by the magnet. It probably acts merely mechanically, for it 

 is to be noted that those acids which evolve a gas other than 

 hydrogen (perchloric acid for instance), which is not absorbed 

 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 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 mag- 

 netic 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. Nickel 

 and cobalt tend to act like iron, though they are attacked 

 with such difficulty that the phenomena are much less strongly 



