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THE first estimations of phosphate in sea-water, in which sufficiently accurate analytical methods .were adopted, were carried out more or less simultaneously by Matthews (1916Matthews ( , 1917 and by Raben (1916-20) working independently. Matthews used the colorimetric method of Pouget and Ohouchak (1909, 1911), and precipitated the phosphate from 500 C.c. of sea-water by means of ferric chloride. Pouget and Ohouchak checked their method using the gravimetric method of Posternak, weighing the barium phosphomolybdate obtained. Values similar to those obtained by Matthews were obtained in the Government Ohemist's Laboratory, London, upon samples sent from Plymouth, and the seasonal variation in the phosphate content of sea-water as found by one of us (1923) is in close agreement with the analyses recorded by Matthewsalthough an entirely different method was used, that of Deniges, which is colorimetric.Raben analysed sea-water from many sources and from various depths, but his results and those of his co-workers are uniformly much higher than those of Matthews andAtkins (1925, 1926) for surface water; moreover, these results as plotted by Brandt (1920) never indicate that lack of phosphate may set a limit to the algal plankton, for the minimum value found was close to 50 mg. of phosphate, as P 205per m.3, whereas Matthews and Atkins found the surface water to be almost entirely devoid of phosphate in summer. As has previously been pointed out by one of us (1926) a serious discrepancy exists, though Raben tested his analytical methods upon known minute amounts of phosphate in the presence of the salts of sea-water, and his results leave nothing to be desired so far as their accuracy is concerned and within the limits of the conditions of his test.Matthews, however, found that with the ammonium phosphomolybdate precipitate a little molybdic acid was liable to be thrown down, and he considered the colorimetric method to be the preferable. He also found

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