some Copper Salts in Aqueous Solution. 339 



Conclusions. 



It is at once noticeable that the curves representing the 

 absorption-ratios of all the solutions appear to start out from 

 the same point at the red end of the spectrum and for a short 

 distance to follow an identical course, diverging more and 

 more widely as the wave-length decreases. It appears natural 

 to attribute this absorption in the red, which is common to 

 all the solutions, to the common constituent copper, and to 

 attribute the differences partly to the absorption exercised 

 by the acid radical itself, and partly to the influence which it 

 has on the vibrations of the copper. Something analogous to 

 this influence of the acid radical on the absorption of the 

 copper was observed by Abney and Festing* in their work 

 on the absorption-spectra of organic and other compounds in 

 the infra-red. They found that a number of definite lines in 

 these spectra undoubtedly were due to hydrogen, but that 

 the number of them and their relative intensity in any spec- 

 trum were dependent on the other atoms with which the 

 hydrogen was combined. The investigations of Hartley, 

 Kriiss, Vogel, and others on the absorption-spectra of organic 

 compounds have shown that the absorption of light by a sub- 

 stance is dependent on its chemical constitution. Every change 

 in the nature, number, or linking of the atoms causes a corre- 

 sponding change in the absorption-spectrum. So that in general 

 when two substances are chemically combined the absorption- 

 spectrum of the compound will be different from that of the 

 constituents. On the other hand, when two substances which 

 are not chemically united exist together in a solution, the 

 absorption-spectrum is the sum of the two separate spectra. 

 Arguing backwards, it seems probable that, if two substances 

 in a solution absorb light independently of one another, so 

 that replacing one of them by something else makes no 

 difference in the absorption-spectrum of the other, then they 

 are not chemically combined. In the dilute solutions of the 

 three copper salts examined this appears to be — at least very 

 nearly — the case. Replacing S0 4 by Cl 2 or by (N0 3 ) 2 makes 

 very little difference in the absorption, though in the strong 

 solutions the difference made by such a replacement is very 

 large. One would, therefore, seem to be forced to the 

 conclusion that only in the strong solutions is the copper 

 really chemically combined with the acid part of the salt, and 

 that as the solutions become more and more dilute the dis- 

 sociation of the salt becomes greater and greater. It has 

 already been pointed out at the beginning of this paper that 



* Phil. Trans. (1881), Part III. p. 887. 



