The relative intensities of monochromatic ultra-violet light determined throughout the ultra-violet mercury spectrum by the use of a fluorescence photometer employing widely different chemical substances were shown to check the intensities of the light given by the thermopile. The agreement between the photometric and thermopile measurements indicates that both methods may be employed for measuring accurately the energy in the ultra-violet region.

The usual direct methods for measuring radiant energy employing either bolometer or the thermopile are non-selective, so that any one desirous of evaluating the energy of a part of the spectrum must use suitable filters, and obtain the result by difference. Coblentz* 1 has had considerable success with this method in measuring the ultraviolet, employing his very sensitive bismuth-silver vacuum thermopile-galvanometer set, glass filters and quartz water cells to absorb the undesired radiation. The authors have successfully used a similar procedure substituting for the very sensitive, but fragile, bismuth-silver pile one of copper-constantan designed and constructed by Coblentz. This pile while less sensitive than the bismuth-silver pile is much better adapted to ordinary use.The thermopile-galvanometer set, however, required considerable attention and frequent calibration. Results frequently had to be discredited because of vibration and magnetic disturbances within the laboratory. Consequently, it was advisable to devise, if possible, other moderately accurate methods for measuring ultraviolet radiations. A radiometer sensitive to ultraviolet light alone would be the ideal. A study of photoelectric cells failed to suggest any possibilities in this field. The quest, therefore, turned to the realm of photochemistry.A photochemical reaction to be acceptable must consist of a simple, definite reaction with an easily measurable and controllable reaction velocity. After much deliberation and experimentation it was decided that the photochemical decomposition of aqueous oxalic acid solution sensitized by uranyl sulfate had possibilities. This reaction had been previously investigated by Mathews and Dewey2 who found that varying the amounts of uranyl salts affected the rate of oxalic acid decomposition. The reaction was rapid and could be very readily followed by means of titration with permanganate solution.The preliminary experiments were designed to study the character of the photochemical reaction itself without considering the energy consumption and distribution.Apparatus.-The light sources employed were air-cooled quartz mercury-vapor arc lamps operated on 110 volts d. c. with a burner consumption of 75 volts and 5 amperes.

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