156 On a Practical Constant- Volume Air- Thermometer. 



all the tubes are heated with a Bun sen flame very nearly to 

 the melting-point of the glass*. When it is perfectly certain 

 that there is nothing but pure dry air in the bulb and tubes, 

 these are allowed to cool with free passage to the atmosphere 

 through the drying-tubes. The bulb is then surrounded with 

 broken ice, and the three-way stop-cock t' is opened. The 

 liquid of the volume-gauge now finds its level ; and, noting 

 the barometer roughly (merely to know approximately the 

 pressure), I seal the tail-piece at the extremity. The bulb 

 now contains about the quantity of air required, and it is 

 only necessary to remove the tail-piece. For this purpose 

 the ice is taken away, and the liquid of the gauge is once 

 more drawn back to a considerable extent, thus making a 

 partial vacuum to avoid blowing out of the air during sealing. 

 The blowpipe-flame can then be applied, and the sealing 

 finished off as in fig. 2. Finally, the manometer and pressure- 

 apparatus are connected to the volume-gauge, and the con- 

 stant of the instrument is obtained by determining the 

 pressures required, including the barometric pressure, to 

 bring the liquid of the volume-gauge into the marked position, 

 first at the temperature of melting ice, and then at the 

 temperature of steam at normal pressure. When reading the 

 standard barometer, I also, in accordance with a most con- 

 venient suggestion by Professor Quincke, read at the same 

 time my standard aneroid ; and this for most purposes, with 

 occasional comparison with the standard mercurial barometer, 

 is amply sufficient to give the barometric variations. As in 

 the case of the mercurial thermometer, so also in the air- 

 thermometer there is sure to be a secular contraction of the 

 bulb ; and, with the large bulbs used for the air-thermometer, 

 it is quite possible that the redetermination of the constant of 

 the air-thermometer from time to time may be necessary. 



Convenient formulse for calculating temperatures from the 

 indications of the air-thermometer are easily obtained. Such 

 formulae were given by Jolly (Jubelband von PoggendorfFs 

 Annalen) , who also made fresh determinations of the expansion 

 of air and other gases. Some of these formulae are quoted in 

 the Leitfaden der Praktischen Physik of Kohlrausch ; but 



* By this process every trace of moisture and condensed air is driven 

 up from the walls of the tube ; and, the bulb being filled with perfectly 

 dry air, it seems certain, from the experiments of Bunsen and from some 

 which I have myself carried out, that there is no subsequent per- 

 ceptible condensation of air at the surface of the glass, such as has some- 

 times been supposed to vitiate the readings of the air-thermometer. Air 

 only condenses on the surface of the glass when there is moisture present 

 — at any rate in such quantity as would be perceptible in a case like the 

 present. 



