Intelligence and Miscellaneous Articles. 457 



complete and necessarily overlaps to some extent the article on 

 this same subject in the companion 'Dictionary of Applied 

 Chemistry,' edited by Prof. Thorpe. It is interesting to notice 

 as an illustration of the inseparable union between theoretical and 

 applied chemistry that the theory of the constitution of the naphtha- 

 lene derivatives is more completely discussed by Mr. Wynne in 

 Thorpe's Dictionary than by Dr. Morley in the present article. 

 Among other lengthy contributions under this letter are two by 

 Mr. Muir on Nitrogen and the Nitrogen Group of Elements, 

 which together occupy about 20 pages. The article on the 

 Periodic Law by Mr. Douglas Carnegie is also noteworthy as an 

 admirable statement of this subject. 



Enough ha3 been said of the contents of the present volume to 

 show that it is as full of interest to chemists as its predecessors, 

 and the editors are again to be congratulated on the results of 

 their labours. We have tested the work over and over again by 

 referring to those topics with which we happen to be personally 

 familiar, and have found it in no way behind its companion volumes 

 in completeness and accuracy. Among the contributors whose 

 articles we have not hitherto mentioned are Dr. Halliburton, who 

 writes on Milk and Muscle; Dr. Eideal, who contributes the 

 articles on Paraffin and Petroleum ; Mr. Shenstone, who treats of 

 Ozcne ; and Dr. Tilden, who writes on Pentinene. Some of the 

 present contributors have not before appeared in the pages of the 

 new Dictionary, but their names indicate that the editors have 

 known where to look for authoritative treatment of the respective 

 subjects. 



LTV. Intelligence and Miscellaneous Articles. 



ON THE COLOUR OF IONS. BY W. OSTWALD. 



IT follows from the dissociation hypothesis of Arrhenius that the 

 absorption-spectrum of a very dilute solution of a salt is the 

 sum of the spectra of the ions. If one of the ions be so chosen that 

 it exerts no absorption in the region investigated and is therefore 

 colourless, the colour of all the salts formed of one coloured ion 

 and several colourless ions must be identically the same. This 

 conclusion has been tested and confirmed by the author on a series 

 of salts, both by actual observation and by photographs. The 

 photographs were so arranged that the spectra of one series of 

 salts were on a plate under each other, so that they could be con- 

 veniently compared. 



The following salts were investigated : — 



1. Permanganates. — The solutions investigated were prepared by 

 mixing a decinormal solution of barium permanganate with an 

 equal volume of an equivalent solution of the desired metal, and 

 then diluting with 50 times as much water. The final solutions 



