657 



mjself of this opportunitj. I am liappj to express my sincere tiianks 



to liiin on (iiis occasion for his assistance 

 )k<i in the investigation. ') 



§ 2. Details of the determinations. Tlie 

 temperatures were measured with the helium 

 tiiermometer described in Comm. NM52a and 

 were reduced in the same way as on that 

 occasion to a scale wiiich approaches tl)e 

 absolute scale as closely as possible (Avo- 

 gadro-scale of helium according to a future 

 communication by Cath and Kamkrlingh 

 Onnes). 



The vapour-pressure apparatus aoreed in 

 its main featui-es with model A used by 

 ' Kamerlingh Onnes and Braak (Gomm. N". 

 107a), in particular I adhered to the use 

 of the copper tube the purpose of which is 

 to prevent a lower tempei'ature existing 

 anywhere in the apparatus but in the bulb 

 where the liquetied gas collects, at which 

 spot the temperature of (he bath is measured. 

 As in the previous experiments mentioned 

 use was made of the advantage offered by 

 the apparatus of allowing different quantities 

 of gas to be condensed in the bulb. As 

 regards the manometer the modification was 

 adopted which Holst introduced in his 

 vapour-pressure measurements of methyl 

 chloride (Comm. N". 1446 Sept. 1913): the 

 moveable tube connected to the fixed tube 

 by a rubber tube is replaced (see fig. 1 *) 

 by a fixed manometric tube') to which a mercury vessel is attached 

 by means of a rubber tube. This fixed tube can be exhausted or it 



^) Part of the present paper is embodied in P. G. Cath, Dissertation Leiden 

 1917, which appeared a few months after the presentation of this communication 

 (see also note 1). 



2) The letters are for the greater part the same as in fig. ^4 Comm N'\ 107a, to 

 which we may refer for tiie further description; a more complete explanaiion than 

 that given above seems unnecessary. 



*) The protection from temperature-changes was obtained by packing in wool. 

 The temperature of the mercury column was determined by means of thermometers 

 suspended beside the tube at the upper and lower ends of the mercury column. 



43 



Proceedings Royal Acad. Amsterdam. Vol. XXI. 



