.wherry: neodymium, cause of red-violet color 143 



poor so that there is any large excess of ferrocyanide in contact 

 with the electrode before the zinc is all precipitated, the end 

 point can be suppressed entirely; or, if by various devices the 

 titration is carried on so that the electrode is always in contact 

 with small concentrations of both zinc and ferrocyanide ion, the 

 point of interception of the normal and abnormal curve can be 

 moved almost to the beginning of the titration, the titration 

 curve thus having a very flat rather than the usual sharp maxi- 

 mum. Of course, in all these cases, except in the case of no 

 stirring, the end point is perfectly sharp and reproducible, and 

 under any of these varied conditions this method of determining 

 the end point is much more rapid, convenient, and accurate 

 than those employing the usual indicators (iron or uranium). 



All these phenomena seem to show that in cases of this type 

 depolarization can be accomplished only by some agent (in this 

 case ferrocyanide, in Forbes and Bartlett's case, ferrous iron) 

 whose oxidation and reduction reactions on the platinum electrode 

 are reversible, and that the persistent irreversibility of the 

 Forbes and Bartlett curve and of our curve was due simply to 

 the fact that the only substance in the solution capable of rapid 

 reversible reaction at the electrode was removed from the possi- 

 bility of acting — by precipitation in our case, and by the excess 

 of an irreversible strong oxidizing agent in Forbes and Bartlett's 

 case. This explanation is independent of any theory of the 

 cause of polarization 



MINERALOGY. — Neodymium as the cause of the red-violet 

 color in certain minerals. 1 Edgar T. Wherry, U. S. Na- 

 tional Museum. 



Impressed by the intense red-violet color shown by the per- 

 manganates, many chemists are inclined to interpret the similar 

 colors occasionally shown by normally colorless minerals, such 

 as calcite and apatite, as due to the presence of manganese. 

 Of course, permanganates may be at once excluded from con- 

 sideration, for they can only be produced by a somewhat vio- 



1 Published by the permission of the Secretary of the Smithsonian Institution. 



