219 



as reciiiired, by nieans of tlio piiicliiiig' sltcvv^ A'^ and A'^. The leiii- 

 peratiire of the jacket is read on the tiieniionieter 7^ 



The vapour pressure tube A' uhit-li is sealed to the nianonieter 

 tube, bent over and closed at the bottom, is placed in a cryostat 

 with a calibrated resistance thernioineter which gives the temperature 

 of the evaporating' surface. The diametei- of the tube A* was J. 71 cm. 



The best method of securing an even temperatui-e in the cryostat 

 is stirring with a "pump-stirrer", in this case however, as the smallest 

 vibrations disturb the readings of the manometer, preference was 

 given to the use of a stream of vapour-bubbles evolved by local 

 heating with an electrically heated wire. 



§ 3. Calibration of the absohite manometer. In the first |»lace it was 

 necessary to consider for what range of pressures the manomelei- 

 may be used as an absolute instrument. In the second place seeing 

 that the distance of the mica-plate to the windows V cannot be 

 treated as infinitely small and the sensibility can thus only be 

 calculated approximately, the value of a scale reading at given 

 difference of temperature between outer wall and mica plate must 

 be expressed in baryes. 



According to Knudsen the force per cm' K on the plate in the 

 limiting case, when the mutual collisions of the molecules may be 

 disregarded, is given by 



where p stands tor the pressure, 7\ and 7\ for the temperatures 

 of plate and wall respectively. 



As regards the condition under which this formula may be used, 

 it is found that in oj-der to reach an accuracy of Va Vo i" the final 

 result, the free path must be at least ten times the diameter of the 

 tube. The range of the manometer is thus different according to the 

 gas used. For carbon dioxide according to Bkeitenbach's measurements 

 of the viscosity at 20° p.l = 7 A, if p is expressed in baryes and A 

 is the mean free path in cms. From this it follows that the range 

 of our gauge reaches up to about 0.3 barye. 



The range can also be determined by connecting the manometer 

 with spaces, where known pressures are established, and ascei'taining 

 for what pressures the elongation remains proportional to the pres- 

 sure. In any case it is necessary, with a manometer such as ours 

 which, as we have seen, does not allow the calculation of the 

 reduction factor from the dimensions of the apparatus, to make 

 measurements for the determination of this factor. By extending the 



