HEAT. 



(3), also for air, Oallendar has calculated that the differences are as in 

 the following table (Phil. Trans., A., 1887, p. 179): 



A gas thermometer has a very great advantage over a mercury glass 

 thermometer, in that the expansion of the containing vessel has a com- 

 paratively small effect, and an approximate knowledge of its expansion 

 suffices to give the necessary correction unless the temperature be high. 

 The gas, if pure, will always behave in the same way, and therefore 

 the same values should be obtained for a given temperature with different 

 instruments, and direct comparisons of the instruments should not be 

 necessary. The methods used by Regnault for the measurement of 

 expansion at constant pressure, and for pressure increase at constant 

 volume between 0. and 100 0., illustrate the two types of instrument. 

 A study of Fig. 36 will show at once that, though the gas scale has 

 advantages, its practical use has serious disadvantages. The apparatus 

 is bulky. It is not " direct reading," i.e. the temperature is not at once 

 read off, but manipulation is required, and calculation must be made 

 from the measurements taken. Corrections too must be made for the 

 expansion of the gas reservoir, and for the gas in the tube connecting 

 the reservoir to the manometer. As the temperature of this gas is 

 different at different points, the correction for it is uncertain. 



Regnault's Normal Air Thermometer. Regnault's researches 

 first made exact gas thermometry possible. He employed a thermometer 

 of the constant- volume principle, the bulb containing dry air freed from 

 carbonic acid, and with the pressure at C., equal to 1 atmosphere. 

 The instrument is represented by Fig. 36. The mercury was always 

 brought to the same point on the bulb side of the manometer by adding 

 mercury on the open side, or running it out at the tap below, and 

 the barometric height + or the difference of level on the two sides 

 of the manometer gave the pressure of the air in the bulb. He termed 

 this instrument " the normal air thermometer." 



A simple form of the instrument, devised by Jolly, is represented 

 in Fig. 37. 



The two limbs of the manometer are connected by a flexible tube t, 

 and one side, M', is movable up and down. On the other side, M, is an 

 index mark at a, to which the mercury is always brought before the 

 pressure is measured. At S is a screw by which the bulb and connect- 

 ing tube can be detached, and after being thoroughly dried they can 

 be connected up again. R is a three-way tap either putting the bulb 



