The valve H (fig. 25) is used to cut off the tension-equalizer completely from the rest of the system at the exact moment of the end of the experimental period. After the motor has been stopped and the slight amount of air partly compressed in the blower has leaked back into the system, and the whole system is momentarily at equal tension, a process occupying some 3 or 4 seconds, the gate-valve H is closed. Oxygen is then admitted from the pet-cock K until there is a definite volume in J as measured by the height to which the diaphragm can rise or a second pet-cock is connected to the can I and a delicate petroleum manometer attached in such a manner that the diaphragm can be filled to exactly the same tension each time. Under these conditions, therefore, the apparent volume of air in the system, exclusive of the tension-equalizer, is always the same, since it is confined by the rigid walls of the calorimeter and the piping. Furthermore, the apparent volume of air in the tension-equalizer is arbitrarily adjusted to be the same amount at the end of each period by closing the valve and introducing oxygen until the tension is the same. BAROMETER. Recognizing the importance of measuring very accurately the barometric pressure, or at least its fluctuations, we have installed an accurate barometer of the Fortin type, made by Henry J. Green. This is attached to the inner wall of the calorimeter laboratory, and since the calorimeter laboratory is held at a constant temperature, temperature corrections are unnecessary, for we have here to deal not so much with the accurate measurement of the actual pressure as with the accurate measurement of differences in pressure. For convenience in reading, the ivory needle at the base of the instrument and the meniscus are well illuminated with electric lamps behind a white screen, and a small lamp illuminates the vernier. The barometer can be read to 0.05 millimeter. ANALYSIS OF RESIDUAL AIR.