some Copper Salts in Aqueous Solution. 341 



copper salt would lead (as it does with chromium salts) to a 

 separation of hydroxide. The small amount of precipitate 

 which forms in solutions of cupric salts after standing for some 

 time appears to be due to impurities in the water (chiefly 

 ammonia and carbon dioxide). 



(3) According^to the theory of Armstrong* and Traube "f, the 

 changes in the absorption- spectra which occur on passing from 

 concentrated to dilute solutions are due to the larger aggre- 

 gates of molecules which exist in the concentrated solutions 

 breaking up into smaller molecules. According to this theory, 

 the molecules of the salt, even in the most dilute solutions, 

 still exist as such ; and these dilute solutions might therefore 

 be expected to show characteristic differences, similar to those 

 observed in the strong solutions. This at any rate in the case of 

 cupric salts is not the case in the visible part of the spectrum. 



(4) The changes of colour which the solutions of copper 

 chloride exhibit when its solution is diluted have long been 

 regarded as due to the formation of different hydrates. If 

 we admit with Pickering the existence of hydrates in dilute 

 solutions containing 1000 or more molecules of water, it is 

 conceivable that two salts of the same metal with colourless 

 acids should have in dilute solution the same spectrum, for 

 the influence exercised on the vibrations of the metal by the 

 acid radical would become negligible compared with that 

 exercised by the 1000 water molecules. Two compounds 

 such as CuCl 2 + 1000H 2 O and GuS0 4 + 1000H 2 O might well 

 have the same spectrum. The experiments are hardly com- 

 plete enough to allow of any very definite conclusion on this 

 point, but as I am occupied with further experiments on the 

 absorption-spectra I hope to return to it on another occasion. 



The large changes of colour which often occur on diluting 

 the solutions of the halogen salts of heavy metals may perhaps 

 be connected not with hydrates but with the halogen. Glad- 

 stone | has pointed out that the spectra of strong solutions of 

 the bromides of platinum, gold, copper, and of potassium pal- 

 ladium bromide, all appear to be made up of the absorptions 

 of bromine- water and of a salt of the metal with a colourless 

 acid. It appears as though the halogen when combined with 

 a metal exercised its absorption in the same way as in the 

 free state, but modified to a greater or smaller extent by the 

 metal with which it is combined. 



The following is a summary of the results arrived at : — 



* J. Cliem. Soc.liii. p. 116 (1888). 



t Berichte, xxiii. p. 3582 (1890). 



\ Gladstone, Phil. Mag. [4] xiv. p. 418 (1857). 



