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IN a series of papers emanating from these laboratories during the past five years unequivocal spectroscopic data for vitamin A have been established, and the ground has been cleared for a final comprehensive attack on the problem of isolating the vitamin and establishing its constitution. The present communication embodies the results of work carried out during the last 18 months, and leads us to the view that a definite solution of the problem is now in sight, a view supported by the recent highly important and illuminating publications of Karrer, Morf and Sch6pp [1931, 1, 2], to which further reference will be made later. Vitamin A, both in liver oils and concentrates, exhibits a broad continuous absorption band in the ultraviolet , extending from 260 to 380 m,u, and exhibiting a maximum near 328 m,. The intensity of this band can easily be measured and used as a definite quantitative test for the potencies of oils and concentrates. Vitamin A also shares with a variety of polyene substances the power of giving a blue solution with a saturated solution of anhydrous antimony trichloride in chloroform. This colour test gains in specificity when the spectral absorption of the blue solution can be accurately described. As a matter of convenience attention may be concentrated on the points of maximum absorption described in terms of intensity and spectral location'. Thus the blue solution obtained with vitamin A exhibits two maxima at 617 and 583 mu respectively, the former being generally much more intense than the latter, which it tends to mask. In unsaponified oils the maxima are displaced to 606 and 573 m, respectively, and here the longer wavelength absorption 1 The notation used in this paper is necessarily a little different from that employed in spectroscopic studies on pure substances. The usual method is to state the maximum molecular extinction coefficient e, defined by E =log Io/I =ecd. (Io is intensity of incident light, I is intensity of emergent light, E is extinction, c = molar concentration, d = thickness in cm.) In this work El, refers to log IO/I at a wavelength maximum, using a solution containing 1 % of vitamincontaining substance and a 1 cm. cell.

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