226 



vapour pressure tubes. They were partly doublevvalled ; the walls 

 are sealed together at the tO}) and free at the bottom. 



Where the external tube ends, the inner tube becomes narrower 

 and is bent round in a spiral finishing up in a small bulb ; this 

 bulb is placed at the level of the middle of the thermometer. In 

 the space between the two walls of the tube a wire is inserted 

 which can be electrically heated, so as to secure, that the bulb is 

 the coolest place of the vapour pressure tube. The object of the 

 spiral is to compel the molecules which come in from above to 

 collide a number of times with the wall before reaching the bulb. 

 That part of the tube cannot contribute to the thermal molecular 

 pressure, as it is practically at the same temperature over its 

 entire length. 



We have made a series of observations for carbon dioxide with 

 this apparatus ^). 



We confine ourselves here to those measurements which give at 

 the same time a check on the last measurements with the absolute 

 manometer contained in Table 2. The results were as follows: 



The last value will be seen to agree well with those obtained 

 with the absolute manometer, while the tirst appears a little too 



1) The measuiemenls at liiglier lompcratiiros arc dealt with in the next com- 

 municalion : S Weber The vapour pressures of carbon dioxide between — liCG. 

 and — 16U°C. 



