=The Constant Volume Gas Thermometer.=—In applying the properties of a gas to practical temperature measurement, we may devise some means of determining the increase in volume when the gas is allowed to expand, or the increase in pressure of a confined gas may be observed. The latter procedure is more convenient in practice, and the instrument used for this purpose is known as the constant volume gas thermometer, one form of which is shown in fig. 1. The gas is enclosed in a bulb B, connected to a tube bent into a parallel branch, into the bend of which is sealed a tap C, furnished with a drying cup. The extremity of the parallel branch is connected to a piece of flexible tubing T, which communicates with a mercury cistern which may be moved over a scale, the rod G serving as a guide. In using this instrument the bulb B is immersed in ice, and the tap C opened. When the temperature has fallen to 0° C., the mercury is brought to the mark A by adjusting the cistern, and the tap C then closed. The bulb B is now placed in the space or medium of which the temperature is to be determined, and expansion prevented by raising the cistern so as to keep the mercury at A. When steady, the height of the mercury in the cistern above the level of A is read off, and furnishes a clue to the temperature of B. If the coefficient of pressure of the gas used (in this case, air) be known, the temperature may be calculated from the equation P_{1} = P_{0}(1 + _bt_), where P_{1} is the pressure at _t_°; P_{0} the pressure at 0°; and _b_ the coefficient of pressure; that is, the increase in unit pressure at 0° for a rise in temperature of 1°. Thus if P_{0} = 76 cms.; _b_ = 0·00367; height of mercury in cistern above A = 55·8 cms.; then P_{1} = (76 + 55·8) = 131·8 cms., and by inserting these values in the above equation _t_ is found to be 200°. In the instrument described, P_{0} is equal to the height of the barometer, since the tap C is open whilst the bulb is immersed in ice. The coefficient of pressure may be determined by placing the bulb in steam at a known temperature, and noting the increased pressure. In the equation given, P_{1}, P_{0}, and _t_ are then known, and the value of _b_ may be calculated. [Illustration: FIG. 1.—CONSTANT VOLUME AIR THERMOMETER.] In using this instrument for exact determinations of temperature, allowance must be made for the expansion of the bulb, which causes a lower pressure to be registered than would be noted if the bulb were non-expansive. Again, the gas in the connecting tube is not at the same temperature as that in the bulb; an error which may be practically eliminated by making the bulb large and the bore of the tube small. The temperature of the mercury column must also be allowed for, as the density varies with the temperature. When the various corrections have been made, readings of great accuracy may be secured.