ACTION OF A MAGNET ox CHEMICAL ACTION 249 



greater than elsewhere, such points will be protected, while if the force 

 is sensibly constant over the surfaces exposed there will be no protection 

 at any point. With all the forms of experimentation tried this law 

 held without exception. It therefore appears that the particles of 

 magnetic material on which the chemical action could take place are 

 governed by the general law of magnetic attraction and are held in 

 place against chemical energy precisely as they would be held against 

 purely mechanical force. To sum up: 



When the magnetic metals are exposed to chemical action in a 

 magnetic field such action is decreased or arrested at any points where 

 the rate of variation of the square of the magnetic force tends toward 

 a maximum. 



It is quite clear that the above law expresses the facts thus far 

 obtained, and while in any given case the action of the magnet is often 

 complicated by subsidiary effects due to currents or by-products, the 

 mechanical laws of motion of particles in a magnetic field hold here as 

 elsewhere and cause the chemical action to be confined to those points 

 where the magnetic force is comparatively uniform. 



The effect of currents set up in the liquid during the action of the 

 magnet cannot be disregarded especially in such experiments as those 

 of Xichols (this Journal, xxxi, 272, 1886) where the material acted on 

 was powdered iron and the disturbances produced by the magnet would 

 be particularly potent. The recent experiments of Colardeau (Journal 

 de Physique, March, 1887) while perhaps neglecting the question of 

 direct protection of the poles, have furnished additional proof of the 

 purely mechanical action of the magnet by reproducing some of the 

 characteristic phenomena where chemical action was eliminated and 

 the only forces acting were the ordinary magnetic attractions. 



An attempt was made to reverse the magnetic action, i. e. to deposit 

 iron in a magnetic field and increase its deposition where there was a 

 sharp pole immediately behind the plate on which the iron was being 

 deposited. This attempt failed. The action was very irregular and the 

 results not decisive. The question of stirring effect was also examined. 

 Usually stirring the liquid about one pole increased the action on that 

 pole, but sometimes produced little effect or even decreased it. This 

 however is in entire agreement with the irregular action sometimes 

 observed in the case of the after-effect in the original experiments. 



An excellent method of experiment is to imbed an iron point in wax 

 leaving the minute point exposed: imbed a flat plate also in wax and 

 expose a point in its centre. Place the point opposite to the plate, but 



