(79) 



into it is given only schematically in the ligiire. In the same manner 

 as the tnbe a^ the thermoelement is easily introdnced and moved 

 up and down through a tube arranged for this end. 



The controlling rod for determining the mean temperature of (he 

 capillar}' of the thermometer or piezometer coidd be omitted in 

 these measurements. 



§ 2. The working of the cryostats described in the last section 

 and in former communications is based upon the cooling caused b^' 

 evaporation at the surface of the liquid. Although the temperature 

 in those apparatus is almost everywhere uniform there yet remains 

 a colder layer at the surface and a warmer one at the bottom. 

 In some measurements it is very disturbing that the temperature 

 at the top of the bath is somewhat, though very little, lower than 

 elsewhere. In a following communication I hope to be able to give 

 drawings of a cryostat where the bath is surrounded from the bottom 

 upwards by vapours of a lower temperature than that of the bath, 

 so that if we regulate the pressure there is a continual heating 

 instead of a continual cooling at the surface and the normal condition 

 that the temperature of the upper part of the bath is higher, is reached. 



Physics. — "Methods and apparatus used in the cryogenic labora- 

 tory. VIII. Cryostat ivith liquid oxygen for temperatures below 

 — 210° Cr Communication N". 94^ from the Physical Labora- 

 tory at Leiden by Prof. H. Kamerlingh Onnes. 



In Comm. N°. 83 IV (March '03) I have described how in one 

 of my cryostats constant temperatures between — 195° C. and 

 — 210° C. (in round numbers) are maintained by means of liquid 

 nitrogen. Whereas between — 180° C. and — 195° C. (in round 

 numbers) oxygen is the proper liquid for cryostats, for the range 

 between — 195° C. and the freezing point of nitrogen the latter 

 substance offers the advantage that its vapour pressure is several 

 times lai'ger than that of oxygen and that the quantity of it which 

 evaporates at the same quantity of heat supplied, can be taken up by 

 a vacuumpump of a much smaller capacity. Moreover if for evapora- 

 tion purposes we are obliged to use the same vacuumpump which 

 also serves for the methylchloride or ethylene, the difficulties are 

 much less with nitrogen than with oxygen. All these reasons made 

 us formerly prefer nitrogen for temperatures below — 195° C. For 

 temperatures below the freezing-point of nitrogen, however, we are 



