Absorption Spectra of Dilute Solutions. 



147 



experiments show, the removal of the hydroxide from the solutions 

 leaving them nearly colourless. 



All the solutions in which the hydrolysis was incomplete gave the 

 red colour with ammonium sulpho cyanide. 



The measurements which G. Wiedemann made of the amount of 

 hydrolysis in the more concentrated solutions of ferric chloride appear 

 to be in harmony with the results given in Table XI. Tho numbers, 

 when plotted against concentrations, give two curves, which appear 

 to be parts of the same curve. 



Colloid Ferric Hydroxide, A solution was prepared by dialysing a 

 solution of ferric chloride in which a quantity of ferric .hydroxide 

 had been dissolved. It was clear red-brown by transmitted but 

 apparently fcurbid by reflected light ; it gave no deposit on standing 

 for three months, and passed through filter paper without leaving- 

 any residue. A solution of NH 4 SCN coagulated it, but gave no red 

 colour. Analysis showed that it contained 12'4 per cent, of the 

 amount of chlorine corresponding to the formula FeCl 3 . The extinc- 

 tion coefficients obtained for different solutions, made by diluting this 

 solution, are given in Table XIII ; they are obtained by graphic 

 interpolation from the experimental numbers given in the Appendix, 

 Table VII. 



Table XIII. 



The numbers, on the whole, show some tendency to increase as the 

 dilution increases. A comparison with the numbers obtained for the 

 completely hydrolysed solutions of ferric chloride (last column, 

 Table XIII) shows that the spectra are entirely different. There 

 can be little doubt that both the solutions contain colloid ferric 

 hydroxide. The nitration experiments show that this is the case in 



