.Acxiox OF A MAGNET ox CHEMICAL ACTION 247 



of these the most successful was an acidulated gelatine 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 deflec- 

 tion 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 hydro- 

 gen. The galvanometer was made much more sensitive and removed 

 from any possible disturbing action due to the magnet; and with these 

 precautions the original 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 hydrobromic acid, 

 and then with hydrochloric, hydriodic, sulphuric and others. The 

 strongest observed effect was with hydriodic acid, but as this may pos- 

 sibly have contained traces of free iodine it may be regarded as some- 

 what 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 galvano- 

 meter 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 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 0-0001 to nearly 0-05 volts, never disappear- 

 ing and rarely reaching the latter figure. 



From these experiments it therefore appears that the protective 

 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 be- 

 tween the poles produced by the magnet. It probably acts merely 



