Xxii. an Investigation of the Accuracy of the Barcroft Differential Mano- Meter in Respiration

ALTJHoUGH the Barcroft apparatus is used extensively for the study ofrespiration in biological fluids, we have found no published work in which the accuracy of the results obtained has been submitted to a critical examination. This is somewhat surprising when it is considered that the Barcroft apparatus is employed so frequently for comparative work where the interpretation is often founded on small differences. In view of these considerations and from a knowledge of a certain variability in results from experiments made in triplicate as recommended by Wooldridge and Standfast [1936], it appeared desirable to submit the results obtained by use of the Barcroft technique to a statistical examination. Our work has shown that the use of four, or even of three, manometers usually suffices to give a mean with a reasonably low standard error. For qualitative work serious objection can hardly be raised against duplicate experiments, but a single experiment cannot be recommended, for it may give a dubious result which cannot be checked effectively. The results from the following five series of experiments formed the basis for the statistical work, but use has been made also of the extensive data collected by one of us (W. R. W.) in collaboration with Mr A. F. B. Standfast. In these experiments the usual technique was followed, the manometric cups containing a total volume of liquid of 3 ml. In the following brief description only the active reagents present in the cups are mentioned. Series I. Left-hand cup: Pseudomonas fluorescens; right-hand cup: Ps. fluorescens in M/40 glucose solutions. All apparatus and reagents sterilised before use. Series II. As in Series I, but apparatus and reagents not sterilised before use. Series III. Left-hand cup: Pseudomonas fluorescens + Bacterium coli; righthand cup: Ps. fluorescens+Bact. coli in M/40 glucose solution. Series IV. Left-hand cup: tap water; right-hand cup: crude sewage. Series V. Left-hand cup: activated sludge; right-hand cup: sludge+ crude sewage (Seitz-ffltered). Table I.