110 Messrs. Rowland and Bell on the 



substance evolved hydrogen, it now became necessary to 

 discover and if possible eliminate the cause of the reversal of 

 the current which regularly followed the protective throw. 

 Experiments soon showed that it could not be ascribed to 

 accumulation of decomposition products around the electrodes, 

 and polarization, while it could readily neutralize the original 

 deflection, could not reverse its direction. Whatever the 

 cause, it was one which did not act with any great regularity, 

 and it was soon found that stirring the liquid while the magnet 

 was on uniformly produced the effect observed. Since one 

 pole was simply exposed over a small portion of its side while 

 the other had a sharp projecting point, it was the latter which 

 was most freely attacked when there were currents in the 

 liquid, whether these were stirred up artificially, or were 

 produced by the change in galvanic action due to the presence 

 of the magnet. When the poles were placed in fine sand 

 saturated with acid this reversing action was much diminished, 

 and in fact anything which tended to hinder free circulation 

 of the liquid produced the same effect. Several materials 

 were tried and 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 com- 

 pletely 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, 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 hydro- 

 bromic 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 



