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 photograph, hence the advantage of their great number and equal distribution. 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 determined up to the point of complete transmission following the increase of dilution. The figures obtained were plotted as abscissæ 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 diluting 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 abscissæ. 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