During earlier work carried out in this Institute on carcinogenic mixtures obtained from coal-tar, a study of the fluorescence spectra of such mixtures gave useful indications of the type of compound present, and subsequently assisted in the separation of one of them, 1 : 2-benzpyrene. Very small quantities of material may often be detected by the study of fluorescence spectra, but the results are as a rule only qualitative. In general, at room temperature, the fluorescence spectra of hydrocarbons in solution consist of diffuse bands, and the total emitted energy is often very small and consequently difficult to measure. Moreover, the absolute amount of radiated energy depends on Ike energy distribution in the incident radiation, and on such experimental details as concentration of solutions, and size and shape of containing vessels. On the other hand, a quantitative measurement of the ultra-violet absorption spectrum of a solution of a pure compound can be made relatively easily, and not only furnishes a method of identification of the substance but serves as a basis for a theoretical treatment of its molecular structure.