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room), which is a standardization of the meter circuit. The meter is set for the range from 0 to 100 mw. per square foot, the shutter is opened, and the light intensity is adjusted by means of the lamp rheostat, to bring the needle on the meter to 1.0i.e., 100% of scale. The shutter is closed and opened to ensure that the 0 to 100% scale deflection is obtained.The cadmium sulfide pad containing mercury is placed in the pad-holding tube, heated by means of the movable furnace, and the procedure recommended in the original paper is followed. The data may best be recorded in terms of per cent absorption, which is 100% minus the instrument reading.The curves given in Figures 2 and 3 were obtained by taking known amounts of mercury in 250 ml. of water through the complete procedure. ACKNOWLEDGMENTThanks are here extended to Warren Kamm and Howard Mc-Kinney for assembling the apparatus and preparing the curves. LITERATURE CITED(1) Ballard and Thornton, Ind. Eng. Chem., Anal. Ed., 13, 893 (1941).

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VOLUME 2 2, -NO. 9, SEPTEMBER 1950 1225 comparable chemical analyses. Direct comparison with the amount evaporated was not possible because of the absorption during the necessary mixing period in the closed room.The results deviate, on the average, by 8.3% from the amount found chemically, with the spectrometer low in five out of the seven cases.Although this accuracy is sufficient for many studies in industrial hygiene, particularly when conventional methods are not suitable because of the complexity of the mixture or the need to take instantaneous samples, further work is expected to result in considerable improvement. A further description of this technique and its application will be published U).