INTRODUCTION. 3 



mainly ions and practically no molecules present, it is obvious that in such 

 solutions it is not the molecules which are absorbing light. It must be the 

 ions, since these are the only units present, or something contained within 

 the ions. This was the view of absorption of light introduced by the theory 

 of electrolytic dissociation. 



We have now gone much farther than this. We now know that the ions 

 are not the ultimate units in a solution of an electrolyte. The simplest ion 

 is very complex. It is made up of a large number of electrons, which are 

 unit negative charges of electricity. There is every reason to-day to believe 

 that the electrons are the real absorbers of light, are the units which are 

 thrown into resonance by the various wave-lengths of light. Granting this, 

 there is still a difference between an ion and the atom or atoms from which 

 it was formed. An ion contains one or more free electrons within it or on 

 it, i. e., one or more negative charges than would correspond to the positive 

 electricity within the atom. It would be interesting to know whether the 

 free electron or electrons upon the ion have anything to do with its power to 

 absorb light. This can be tested by studying the absorbing power of mole- 

 cules and then the absorption of light by the ions which are formed when 

 these molecules dissociate. It was with this idea in mind that the second 

 chapter of the work described in this monograph was undertaken. 



A concentrated solution of a salt contains many molecules, and if the solu- 

 tion is sufficiently concentrated there are chiefly molecules and only a few 

 ions present. As the dilution is increased the dissociation increases; the 

 number of molecules becomes less and less and the number of ions greater 

 and greater. The problem, then, is to photograph the absorption spectrum 

 of a very concentrated solution of a salt, the layer being, say, 0.5 cm. deep. 

 Then take the spectrum of a more dilute solution of the same salt; if the 

 dilution is increased 100 times the depth of layer used would be 50 cm. 

 Under these conditions there would be the same number of parts of dis- 

 solved substance in the path of the beam of light; in the second case there 

 would be more ions and less molecules than in the first. By comparing the 

 two spectra we could see whether there is any difference between the 

 absorbing power of ions and molecules, i. e., whether the free electrons upon 

 the ions have anything to do with their power to absorb light. We then 

 took another step, increasing the dilution of the second solution five times 

 and also increasing the depth of the layer of the solution through which the 

 light passed five times, i. e., making the depth 250 cm. This second diluting 

 still further reduced the number of molecules present and increased the 

 number of ions. By comparing the three spectrograms we ought to be able 

 to say whether molecules and ions have the same or different resonance 

 with respect to light-waves; and, if it is different, to point out in what the 

 difference consists. This would then enable us to determine whether the 

 free electrons upon the ions played any part in the absorption of light. 



We shall see that ions have somewhat different absorbing powers from 

 molecules, and in what this difference consists will appear later from the 

 text and from the plates. 



