1894.] On the Absorption Spectra of Dilute Solutions. 287 



tion spectrum. The mean of the numbers obtained for these two 

 salts was regarded as the absorption spectrum of the ion CsH^NC^aO. 

 The solution of dinitrophenol in hydrochloric acid (containing the 

 undissociated molecule C 6 H3(N0 2 ) 2 OH) absorbs very little light ; it is 

 almost colourless. The extinction coefficients of dinitrophenol, and 

 of its coloured ion, being thus known, it was possible to calculate 

 from measurement of the extinction coefficients of a series of solutions 

 of dinitrophenol in pure water, its degree of dissociation in these 

 solutions. The numbers thus obtained were in very satisfactory 

 agreement with the numbers calculated from the electrical conducti- 

 vity of the solutions. 



As an example of the hydrolytic decomposition of a salt in aqueous 

 solution, to the study of which the spectro-photometric method can 

 be advantageously applied, ferric chloride was taken. 



By filtering dilute solutions of ferric chloride through a porous cell 

 all the colloid ferric hydroxide formed can be removed ; and analyses 

 of the solutions before and after filtration showed that the hydroxide 

 formed in solutions containing less than O'OOS gram molecule of 

 FeCl 3 per litre contains no chlorine. The decomposition which occurs 

 in these solutions may thus be most simply expressed by the equation 

 FeCls+3H,0^;Fe(OH) s +3HCl. 



The photometric determinations of the quantity of ferric hydroxide 

 formed in these solutions agreed fairly well with the results of the 

 filtration experiments, though, owing to the difficulty in obtaining 

 the solutions of ferric chloride perfectly clear, they were not so 

 satisfactory as could be desired. 



The quantities of ferric hydroxide formed were not in agreement 

 with the law of Guldberg and Waage, but agreed much better with 

 the modified form of the law due to Arrhenius, in which the electro- 

 lytic dissociation of the different substances is taken into account. 



It was observed that solutions of ferric hydroxide obtained by 

 dissolving ferric chloride in a very large quantity of water, had a 

 different absorption spectrum from that of solutions of ferric hydroxide 

 obtained by dialysis. It is suggested that an explanation of this fact 

 may be found in the differences in the complexity of the molecular 

 aggregates existing in the different solutions. 



Finally, solutions of ferric chloride, to which small quantities of 

 hydrochloric acid had been added, possess such comparatively small 

 power of absorbing light that they cannot be regarded as containing 

 any colloid hydroxide of iron. 



