Absorption Spectra of Dilute Solutions. 



149 



Larger quantities of acid seem to prevent the formation of ferric 

 hydroxide altogether, as the acid solutions could all be filtered 

 through the porous cell without leaving any deposit. The analyses 

 also snowed that their concentration was not diminished by the filtra- 

 tion. 



Table XIV contains the numbers which were obtained with a series 

 of acid solutions. The arrangement of the table is the same as before, 

 only the number of equivalents of free hydrochloric acid added to one 

 equivalent of iron are also given. 



Table XIY. 



These numbers appear to be somewhat irregular, but a glance at 

 fig. 5 will show that this is due to the way the curves cross each 

 other. The curve XXX of the solution containing the smallest 

 excess of hydrochloric acid has a strong resemblance to the curve 

 for Fe(OH) 3 , which is included in the figure for comparison (with 

 ordinates divided by 100). If the whole absorption of solution XXX 

 were due to hydroxide, the amount of hydrolysis would only be -J to 

 ^ per cent, of the salt present. In the curve for the next solution, 

 XXXI, the similarity to that of the hydroxide has disappeared, and as 

 the quantity of acid is increased the curves evidently approach more 

 and more 'towards that of the concentrated acid solution XXXV, 

 which may be taken as representing approximately the absorption 

 spectrum of the undecomposed molecule of ferric chloride. Whether 

 these changes are due to the change of the dissociated ferric chloride 

 into undissociated, or to the conversion of a soluble oxychloride into 

 ferric chloride cannot be decided. It is certain that these solutions 

 contain none of the colloid hydroxide, but they possibly contain a 

 soluble oxychloride. Wiedemann found that solutions of ferric 

 nitrate and sulphate, containing excess of acid, still contain a con- 



