Three methods which have been used to determine nutrient uptake hnetic parameters were compared using steady-state NH,-limited cultures of the chrysophyte Pseudopedinella pyriforrnis. The first 2 methods involved a multiple flask incubation where different concentrations of substrate were added to each flask. Method 1 used a variable incubation time, while the incubation time of Method 2 was short and constant. The third method, the perturbation method, involved one large addition of the substrate to one culture and hence the nutritional past history varied throughout the experiment. This method was used also with the diatoms Skeletonema costatum and Chaetoceros debilis in Si-Limited chemostats. Results inhcate that, for nutrient-limited cultures, kinetic parameters are best estimated using multiple additions of the substrate and a short constant incubation time (Method 2). It appears that this method determines membrane transport, which is still not completely free of feedback inhibition even when the incubation time is very short (e.g. 2 min). The short incubation time is necessary because the maximal uptake velocity (V,) decreases with increasing incubation time especially for phosphate and ammonium. Method 3 provides valuable information on a third parameter, V,, the approximate assimilation rate of the limiting nutrient, that is not obtained by the other methods. Multlple sequential additions of the limiting nutrient to N-or Si-limited Skeletonema costaturn and Chaetoceros debilis revealed that if the additions were small (e.g. 2 pM NH,), there was no change in subsequently determined nutrient uptake kinetic parameters. If the sequential additions were larger (e.g. 6 PM) then the maximal uptake rate slowed with time.

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topic mixtures as test liquids, and indicated that preliminary experiments had been successful.Hawkins suggested a mixture of ethylbenzene and chlorobenzene as an ideal test mixture for columns with up to 80 plates, and also for tests a t both atmospheric and subatmospheric pressures.Its relative volatility ranges from 1.10 a t 760 mm. to about 1.12 at 20 mm. of mercury pressure. The use of an n-heptanemethylcyclohexane mixture a t reduced pressures was also suggested.F. E . Williams of Hercules Powder Go. pointed out the lack of test mixtures composed of compounds other than hydrocarbons. Rossini felt this was because hydrocarbons were more easily obtained in the pure state than other types of compounds. INTERPRETATION OF DATAWilliams pointed out the complexities of analyzing break cuts to obtain over-all composition, and the desirability of being able to calculate composition directly from the boiling point or refractive index curves. Schoenberger recommended the use of the equal-area cut-point method for low temperature distillations, but also indicated this method had definite limitations. Rossini observed that from accurate boiling points and vapor pressuretemperature curves it is possible to calculate the boiling point of an equimolar mixture and thus establish the cut point. The additional difficulties which result from the presence of a third component during the break were noted.Hawkins described the calibration of columns with known mixtures, in order to obtain arbitrary correction factors for locating cut points and account quantitatively for the material subjected to analytical distillation.

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