Absorption Spectra of Dilute Solutions. 



151 



siderable quantity of hydroxide. He has given no measurements on 

 acid solutions of the chloride, but as I found that the acid solutions 

 of ferric nitrate and sulphate behave very much in the same way as 

 the chloride with respect to colour, we may perhaps conclude that 

 all these solutions contain some really soluble substance standing 

 between the undissociated normal molecules and colloid ferric hydr- 

 oxide. 



Some measurements which were made with ferric nitrate and 

 sulphate gave results similar to those obtained with the chloride, but 

 they are very much less complete. 



The following table contains the measurements made on concen- 

 trated acid solutions of ferric sulphate and nitrate, and with a 

 solution of ferric sulphocyanide in ether; owing to the ease with 

 which the latter decomposes the numbers are only approximate, they 

 will serve, however, to give some notion of the relative magnitude of 

 its absorption. 



For comparison the numbers obtained for the concentrated, acid, 

 ferric chloride solution, and for ferric hydroxide are given. 



Table XV. Acid Solutions of Ferric Salts. 



It will be noticed that the different salts of iron all have different 

 absorption spectra, the nitrate is very slightly coloured indeed ; the 

 sulphate brown, and the chloride bright yellow. Ferric hydroxide 

 absorbs several hundred times more light than the salts, and ferric 

 sulphocyanide absorbs nearly twenty times as much light as the 

 hydroxide. 



VOL. LV1I. 



