(With Two Text-figures)

The metal Barcroft respirometer is an accurate and rapidly operated micro-respirometer, suitable for small insects and other animals which can be maintained in an aerial environment for the period of experiment, and is particularly suitable for experiments in which different gas mixtures are employed. It is based on the principle of the differential (Barcroft) type of manometer, but its chief advantages are its small size and construction from a solid brass block. In consequence, the temperature surroundings of the two chambers are kept uniform by the high conductivity of the metal, and gaseous diffusion is very rapid throughout. Into a solid brass block ABCD are drilled two chambers, the experimental chamber (1) being 1 cm. in diameter and 1 cm. deep, the control chamber (2) being 2 cm. in diameter and 1-75 cm. deep. The chambers taper at the base and are continued as fine capillaries i-o sq.mm. in cross-section (EF and GH) to the base of the block, where they terminate in small brass collars. A bent capillary tube R of cross-section 0-2 mm., graduated in cm., containing the manometer fluid, is fitted directly into these collars and sealed with picene wax. The chambers 1 and