COLOR AND CHEMICAL CONSTITUTION 123 



tions. Hartley and Huntington used a cadmium alloy, which they 

 found to give a great number of lines, but in recent work the arc 

 spectrum of iron has been adopted. This latter gives, as already noted, 

 a vast number of lines extending throughout the visible and invisible 

 spectrum in a more or less equally distributed manner. The presence 

 of an absorption band is detected by the absence of lines in the photo- 

 graph, hence the advantage of their great number and equal distribu- 

 tion. By successive photographs accompanying an increase in dilution 

 the greatest degree of absorption, and thus the head of any particular 

 band, may be observed. 



The oscillation frequencies at the edges of these bands were deter- 

 mined up to the point of complete transmission following the increase 

 of dilution. The figures obtained were plotted as abscissae against 

 the corresponding volumes in which definite amounts of the substances 

 were dissolved. The curved lines drawn through these points, called by 

 Hartley the " curves of molecular vibrations," were found to be closely 

 related to the chemical constitution of the compound studied. More 

 recently a better method of plotting results consists in photographing 

 through varying thicknesses of a solution of known strength and then 

 diluting the solution ten times, repeating the observations, again dilu- 

 ting ten times, and so on till the point of complete transmission is 

 reached. The relative thicknesses are now expressed in millimeters 

 of those thicknesses that would be required of the last or most diluted 

 solution, and these values plotted in the form of logarithms as the 

 ordinates over against the oscillation frequencies as abscissae. Curves 

 thus plotted show at once the same relative shift with the ordinates as 

 is made with the thicknesses examined. The persistence of a band, 

 or change of concentration through which a band asserts itself, is 

 well illustrated by this curve. In this point — the persistence — we have 

 a characteristic function of the bands which connects them closely 

 with chemical properties. 



The compounds studied have been entirely within the realm of 

 organic chemistry. In this class we meet with the most pronounced 

 and, at the same time, the most easily varied tints. A study of these 

 variations in color in the closely related organic compounds has, up 

 to the present, occupied the entire attention of investigators, among 

 whom, after Hartley and Huntington, are to be named Baly, Desch and 

 Stewart. 



The absorption spectra in the ultra-violet region may be classified 

 under two broad types; the first type is that in which only a general 

 absorption is present; the second is that in which distinct absorption 

 bands occur, a type usually defined as one of selective absorption. To 

 the first class belongs, broadly speaking, the majority of the aliphatic 

 or open chain compounds; to the second belongs the majority of the 

 aromatic compounds or those of ring structure. Among the first 



