340 Dr. T. Ewan on the Absorption-Spectra of 



in general salts in aqueous solution behave in the same way 

 as these copper salts, though exceptions are known. 



It only remains to compare briefly the results thus arrived 

 at with existing theories of solution. 



(1) According to the theory of Arrhenius the electrolytic 

 dissociation in the most dilute copper solutions which were 

 examined is by no means complete, and therefore the absorp- 

 tion-spectra of the three solutions should not be identical. 



The degree of dissociation is calculated from the numbers 

 of Kohlrausch* for copper sulphate, of J. Trotschf and J. H. 

 van t'HofF and Reicher| for copper chloride, and of Long§ 

 for copper nitrate. The numbers obtained are as follows : — 



CuS0 4 , 0-2856 eq. in 1 1., /i,=344, p m = 1086, 



dissociation = 31*7 percent. 

 „ 2*38 eq. in 1 1., //,= 166 „ =15*3 „ 



CuCl 2 , 0-4189 eq. in 1 1., ^ = 64, ^ = 101, 



dissociation= 63*4 „ 



Cu(M) 3 ) 2 , 0-3552 eq. in 1 1., /i = 65-8, /* w =100?, 



dissociation = 65*8 „ 



It is interesting to observe that the solutions of chloride 

 and nitrate of copper in which the dissociation is far advanced 

 and nearly to the same extent, possess almost identical absorp- 

 tion-spectra, whereas that of copper sulphate is noticeably 

 different, which fact is in agreement with the theory, the 

 dissociation in the copper-sulphate solution being very much 

 less advanced than in the others. 



It is as well to draw attention to the fact that before one 

 can determine whether the limit of dilution after which the 

 absorption no longer changes is identical with the limit at 

 which the dissociation is complete, it may be necessary to 

 examine the absorption in the ultra-violet and perhaps in the 

 infra-red. The experiments in the first part of this paper 

 with copper chloride seem to show that changes go on in the 

 invisible violet after they have ceased in the visible part of the 

 spectrum. 



(2) Knoblauch has explained the alterations in the spectra 

 of certain salts by assuming a hydrolytic dissociation in the 

 more dilute solutions. In the case of copper salts there 

 appears to be no reason for assuming that such a dissociation 

 takes place to any extent. A hydrolytic dissociation of a 



* "Wied. Ann. vi. p. 1 (1879) ; ibid. xxvi. p. 188 (1885). 



t Wied. Ann. xli. p. 259 (1890). 



% Zeitschrift. Phys. Chem. iii. p. 198 (1889). 



§ Wied. Ann. xi. p. 37 (1880). 



