INTRODUCTORY. 3 



nately our knowledge of the parts formed when a salt is dissolved is still 

 very vague. We have methods for measuring dissociation, so we may 

 regard the number of ions and the number of undissociated molecules as 

 known for different conditions. 



Regarding aggregates and solvates, however, our knowledge is very 

 general, indeed. Determinations of molecular weights give some idea con- 

 cerning the existence or non-existence of aggregates, and the methods of 

 Jones and others furnish similar ideas about solvates; but the knowledge 

 gained thus far is not definite enough to enable us to perform experiments 

 along the lines indicated above. A great deal can, however, be learned 

 not only about absorption, but also about the nature of solutions, by the 

 study of absorption spectra under conditions which are varied as much as 

 possible. 



The methods commonly employed are: 



(1) To keep the concentration constant, varying the depth of the cell 

 and photographing the spectra of successive depths one beneath the other, 

 so that the complete spectrogram gives an idea of the intensity of absorp- 

 tion for the different regions of the spectrum, besides locating the absorp- 

 tion bands. 



(2) To keep the depth of cell constant and varying the concentration. 

 The results here should be identical with those obtained by keeping concen- 

 tration constant and varying the depth of cell, provided the solution is of 

 such a nature that Beer's law holds; in general, however, the two methods 

 give quite different results, owing to the change in the nature of the 

 absorbers produced by dilution. 



Another method is that followed in the present work, namely, to vary 

 both depth of layer and concentration in such a manner that the product 

 of the two remains constant. If the nature of the absorbers is not 

 changed by dilution, this method leaves the number of absorbers in the 

 path of the beam of light constant, and hence the spectrum for successive 

 solutions should be identical; or, what amounts to the same thing, the 

 width of the absorption bands as shown by the spectrogram should remain 

 constant throughout. Any deviation from Beer's law would at once be 

 seen by the bands changing in width or position as the concentration is 

 varied. 



A modification of this method, also employed in the present work, is to 

 vary the depth and concentration in such a manner that the total num- 

 ber of ions, or the total number of undissociated molecules in the path of 

 the beam of light remains constant. This is easily done as follows: Let 

 the concentration be denoted by c, the ratio of the number of dissociated 

 molecules to the total number put into solution by x, the depth of solu- 

 tion used by d; the number of ions in a cubic centimeter of the solution 

 is then proportional to ex, and the number of undissociated molecules to 

 c(l x). To keep the number of ions in the path of the beam of light con- 

 stant it is only necessary to keep the product cxd constant; and to keep 

 the number of undissociated molecules constant the product c(l x)d must 

 remain constant. If the successive depths of solution to be used have 

 been fixed arbitrarily, the concentrations are determined in the follow- 



