445 



This value ^) is smaller than that found by direct measurement 

 at the smallest velocity of vaporization")-. 110.2. It is possible that 

 this result indicates that the precautions taken to prevent condensa- 

 tion of the vaporized hydrogen within the calorimeter have not been 

 sufficient. In fact at a velocity of vaporization twice as great as 

 that at which the value mentioned above was found smaller values 

 were obtained, vh. 108.5 and 109.3 at 76.1 and 77.75 cm. pressure 

 respectively. 



Physics. — " Vapour pressures at very low reduced temperatures. 

 II. The vapour pressure of carbon dioxide in the range 

 from — 140° C. to about 160° CV' By Sophus Weber. Com- 

 munication No. 137c from the Physical Laboratory at Leiden. 

 (Communicated by Prof. Kamerlingh Onnes). 



(Communicated in the meeting of September 27, 1913). 



§ 1. In these measurements the heated-wire manometer described 

 before was used^); it was calibrated by the aid of a set of pipettes 

 as shown in fig. 1. The manometer is sealed to the vessel P^ at J 

 and is placed, together with P^ and P^ m a waterbath, in which 

 a temperature of about 20° C. is maintained by means of a thermostat. 

 This temperature is read on a mercury thermometer. The electric 

 connections are the same as in fig. 4 of the first paper. In order 

 to keep back mercury vapour or other vapours which might originate 

 in P, the tube / the volume of which is only 1 7o ot the whole 

 volume was placed in alcohol cooled to about — 100° C. by the aid 

 of liquid air. In a high vacuum the vapours given off b}^ tlie grease 

 of the stopcocks will be condensed in 1. I have tried to prevent 

 this flow of vapour to / while retaining the taps by interposing at 



1) It may be noticed that this value nearly coincides with that calculated from the 

 vapour pressure measurements of Dewar (cf. p. 441 note 1). The difference which 

 shows the value derived from the vapour piessure measurements of Travers and 

 Jaquerod (cf. p. 441 note 1) is due to the diverging course of their vapour 

 pressure cuive, cf. p. 444 note 1. [Added in the translation] 



2) Cf. p. 441 note 1. 



3) H. Kamerlingh Onnes and Sophus Weber, Comm. N". IST/y (June 1913), 



