44 H. A. Rowland and L. Bell — Explanation of the 



which was allowed to harden around the poles. In this case 

 the protective throw was not nearly as large as in the free 

 acid ; since the electrodes tended to become polarized while the 

 gelatine was hardening, and only weakly acid gelatine would 

 harden at all ; but the reversing action completely disappeared, 

 so that, when the magnet was put on, a permanent deflection 

 was produced instead of a transitory throw. 



This point being cleared up attention was next turned to the 

 negative results obtained with acids which attack iron with 

 evolution of hydrogen. The galvanometer was made much 

 more sensitive and removed from any possible disturbing 

 action due to the magnet; aud with these precautions the origi- 

 nal experiments were repeated, it seeming probable that even 

 if the magnetic effect were virtually annulled by the hydrogen 

 evolved, some residual effect might be observed. 



This residual effect was soon detected, first with hydrobro- 

 mic acid, and then with hydrochloric, hydriodic, sulphuric and 

 others. The strongest observed effect was with hydriodic 

 acid, but as this may possibly have contained traces of free 

 iodine it may be regarded as somewhat doubtful. The effect 

 in all these cases was very small, and though now and then 

 suspected in the previous work, could not have been definitely 

 determined, much less measured. 



Some rough measurements were made on the electromotive 

 forces involved in this class of phenomena by getting the 

 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 hydro- 

 gen, up to - 02 or (>03 volts 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 O'OOl to nearly 0"05 

 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 



