Absorption Spectra of Dilute Solutions. . 131 



In some cases the numbers given by two different sets of measure- 

 ments were so nearly the same that they were united to one curve. 

 This was the case with CuS0 4 III and II and part of the numbers for 

 CuCl 2 ; and also with the two solutions of Cu(C 2 H 3 02)2 X and XI. 

 Leaving for the present the numbers for Cu(C 2 H 3 02)2 out of account, 

 it is evident that the differences which occur between the numbers 

 obtained with different solutions of the same salt are as great as 

 those which are found between solutions of different salts. The 

 absorption spectra of the four salts CuSO*, CuCl 2 , Cu(N"0 3 ) 2 , and 

 CuBr 2 must accordingly, in the concentrations used, be regarded as 

 the same. 



In the following table (p. 132) the means of the above numbers 

 are compared with the numbers which I formerly obtained (' Phil. 

 Mag.' [5], vol. 33, p. 317, 1892). 



These numbers show that the absorptions change very little when 

 the solutions are diluted. The electrolytic dissociation, however, 

 changes considerably. For CuSO* the fraction of the salt electrolytic- 

 ally dissociated increases from 0'317 in the stronger solution to O494 

 in the more dilute. For CuCl 2 the increase is from 0'634 to O920. 

 That the change in dissociation produces very little change in the 

 absorption is most simply explained by supposing that the Cu exist- 

 ing in chemical combination with other atoms in the solution has the 

 same absorption spectrum as the copper ion. In the case of CuCl 2 

 this leads to a result of some interest. For the concentrated aqueous 

 solution of cupric chloride and also its alcoholic and ethereal solu- 

 tions are brownish-green in colour, and as these solutions doubtless 

 contain the molecule CuCl 2 we cannot ascribe to it the same absorp- 

 tion spectrum as to the Cu ion. In the dilute solutions of CuCl 2 

 examined, therefore, the combined part of the copper probably exists 

 in the form of the ion CuCl. This is also in good agreement with 

 the observation which I made formerly (' Phil. Mag.,' 1892) : that the 

 rapid change in the absorption spectrum which accompanies the dilu- 

 tion of a concentrated solution of CuCl 2 ceases when the concentra- 

 tion reaches a value lying between 0'5 and 0'2 gram molecules per litre. 

 The fractions of the salt dissociated in these concentrations, calcu- 

 lated from the electrical conductivities of the solutions, are 0'41 and 

 0'634 respectively. It would appear thus that on diluting a strong 

 solution of cupric chloride the CuCl 2 molecules first dissociate into 

 the ions CuCl and Cl, and that the dissociation of the CuCl ions only 

 takes place to any considerable extent after this process is tolerably 

 complete, that is, after the dissociation of the salt has reached 50 per 

 cent. This behaviour is very common in the case of dibasic acids, of 

 which a large number have been examined by Ostwald (' Zeitschrif t 

 fur Phys. Chem.,' vol. 3, pp. 186 and 280, 1889). 



It will be seen from Table III that the extinction coefficients of 



