Absorption Spectra of Dilute Solutions. 141 



dilute solutions deposit only ferric hydroxide. He found also that 

 the amount of decomposition increases with rising temperature and 

 increasing dilution. 



Finally, Gr. Wiedemann,* in 1878, published an investigation on 

 the hydrolysis of ferric salts, the determinations of the amount of 

 hydrolysis which had taken place being made by means of the 

 molecular magnetism of the solutions. The molecular magnetism of 

 iron in the form of hydroxide being 0'16 of its value in a strongly 

 acid solution, a measurement of the magnetism of a solution of 

 known strength allows the ratio in which the iron is divided between 

 the two forms to be calculated. 



The fact that, as Krecke observed, the precipitate formed by 

 heating solutions of ferric chloride contains chlorine, and also an 

 observation made by Picton and Linder,f that on filtering a solution 

 of colloid ferric hydroxide containing chlorine through a porous cell 

 the filtrate contained neither iron nor chlorine, led me in the first 

 place to attempt to decide whether the hydroxide formed in dilute 

 solutions of ferric chloride contained chlorine or not. 



For this purpose solutions of known concentration were filtered 

 through a porous cell. The cell was first well washed with distilled 

 water, and then a quantity of the solution filtered through it to re- 

 . move the water from its pores. The next portion of the filtered 

 solution was collected and analysed. The difference between the 

 composition of the original solution and that of the filtrate gives the 

 composition of the substance held back by, the porous cell. The 

 nitration apparently removed the hydroxide completely from the 

 solution. The yellow-brown coloured solutions were, after filtration, 

 either completely colourless or very faintly yellow, according to their 

 concentration. This method of analysis has the drawback that the 

 solution changes its concentration to some extent during the filtra- 

 tion partly owing to evaporation in the vacuous vessel into which it 

 is filtered, partly owing to the" difficulty in removing the water which 

 has served for washing the cell from its pores. A direct analysis of 

 the precipitate is hardly possible as it cannot be collected on a filter, 

 nor can it be washed by decantation, as it refuses to settle. Further, 

 to wash it with water is not allowable, as it would tend to change its 

 composition. The ferric chloride used was purified by repeated re- 

 crystallisation of the hydrate, Fe 2 Cl 6 12H 3 ;J an analysis of its solu- 

 tion showed that it contained iron and chlorine in the ratio repre- 

 sented by its formula. 



* ' Wied. Ann.,' vol. 5, p. 45, 1878. 



f ' Journ. Chem. Soc.,' 1892, p. 152. 



t Bakhuis Eoozeboom, ' Zeit. Phys. Chem.,' vol. 10, p. 477, 1892. 



