222 



pressure tube 7^ was first placed iu liquid oxygen and afterwards 

 in methane of diiferent temperatures. It appeared that the vacuum 

 remained constant within the limits of accuracy and was independent 

 of the temperature of the vapour pressure tube^). Carbon dioxide 

 carefully dried and repeatedly distilled in vacuo was now introduced 

 tlirough a side tube into the tube R, which was cooled with liquid 

 air. When a sufficient quantity had passed over, the apparatus was 

 again connected to the molecular pump and evacuated during a 

 considerable time. The manometer was then sealed off from the 

 pump at a and the pressure measured, while 7^ was surrounded 

 with oxygen boiling under reduced pressure. It was found that the 

 vacuum was the same as before the introduction of carbon dioxide, 

 from which it follows that at — 205° C. the vapour pressure ot 

 carbon dioxide must be smaller than 0,005 barye. A difference ot 

 0,002 barye could be clearly detected and the influence of vibration 

 was also just below this value; 0,005 barye could thus not escape 

 observation. 



[3. x\s a rule the further observations were mpde in the following 

 manner: first the manometer is brought approximately to temperature- 

 equilibrium, and the thermometers 1\ and 7\ are read ; warm water 

 is then made to pass through K^ until a suitable temperature is 

 reached ; T^ and 1\ ai-e again read and the reading on the eye- 

 micrometer is taken. Finally cold water is let into K^. until 1\ has 

 about assumed the original temperature, after which a second obser- 

 vation is made at the same temperature. 



The calculation is made iu the manner explained in section 2. 



As described the vacuum was measured before carl>on dioxide 

 was admitted, It being at tcmi)Gratures between — 160° C. and 

 — 200° C, when 0,007 baryes was found for the pressure. After 

 sealing off the same value 0,007 baryes was found. A complete 

 series of' observations was then taken, first with methane and then 

 with oxygen in the cryostat. The api)aratns was then left standing 

 for a few days, after which a new series of observation was taken. 

 In the mean time the vacuum appeared to ha\c risen to 0,016 

 barye. If this change is taken into consideration the two series 

 agree well with each other. 



In table II the two sets of measurements are combined. 



The figures in the 3"^ column are derived from those in the 2"'^ 



1) The radiation-pressure did not amount in our case to more than 0,05.10—3 

 barye, when the wall was at 100° G. 



