β Scribed by Cole, R. J. ;Bensett, W. H.
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Further tests on a larger (6 1.) scale were then made, using about 3000 counts/min. of la1I. At first only 40-50% recovery of activity was obtained. To improve the yield it was found necessary to take special care to ensure (I) complete precipitation of the silver halides (2) complete retention and even deposition of the precipitate on the centrifugal filter basket and (3) removal of most of the silver chloride which would otherwise cause serious P-ray absorption losses. When all due precautions were taken, the yields finally obtained in a series of esperiments \Yere : 75'!b, 87% 93% 75% 670/L and 90%.
Nevertheless the method has disadvantages. The G.T.1 I is a less sensitive instrument for lniI than the G.M.4, principally because its window absorbs about 80% of the /I-rays from I3lI instead of 15S:,, and contamination by adsorption of other activities on the silver iodide is quite likely to occur. In two esperimenu on 6 1. of river water contaminated with 1200 counts/min. of aged fission products, but containing no radioiodine, 12 and 9 counts/min. respectively were found on the final precipitate. Such contamination is not easily eliminated by any " re-cycling " process.
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## Abstract The possible role of melatonin in melanogenesis was investigated by performing reactions of melatonin with peroxidase + H~2~O~2~ or H~2~O~2~ only, in the presence or absence of UV irradiation. Samples of the reaction mixtures were drawn at different times (from 15 to 480 min), the enzym