50 



always broiiglit to a mark on the stem below the measnring-bnlb, 

 when the pressiire-differeuce was read with a cathetometer, allowing 

 for the height ol' the bafonieter, while the temperatm-e of the 

 waterjacket was observed. 



The oxygen was prepared from pnre potassinm permanganate ') and 

 tlie nitrogen from sohitions of sodinm nitrite and ammonium oldoride '). 

 In the preparation of nitrogen the air was first driven ont by carbon 

 dioxide ; to free the nitiogen from the i-emaining carbon it was 

 frozen ont in liqnid oxygen boiling nnder reduced pressure, before 

 being used. The first part of the evaporating nitrogen was pumped 

 off, and the next part used to fill the apparatus ; the residue was 

 also removed. 



§ 3. Apparatus. 



a. Piezometer. 



The usual type of piezometer used in I he laboratory at Leiden 

 was employed. The volume of the large reservoir was some 500 cm', 

 and that of the small reservoii' about 1 cm'. The form of the latter 

 differed from that used in Comni. N'. 150^ as an other method of 

 stirring was adopted, to avoid the difticullies mentioned there. It 

 consisted of a capillary about 50 cm. long 6', with a bore of 

 + 0.2 mm., and a cylindrical bulb R, about 10 cm. long, which 

 was carefully rounded at the lower end, so that the stirrer ;• con- 

 tained within (a short length of iron wire enclosed in a gla.ss tube) 

 could reach the extreme end, thus avoiding any dead space. The bulb 

 R was graduated; and, as its section and the volume of the stirrer 

 were known by previous calibration, it was possible (o estimate the 

 volume of any litjuid formed in it. The reservoir was calibrated as 

 a whole after its construction had l)een completely finished. 



b. Manometer. 



Pressures were read on the closed hydrogen manometer M. 60'). 

 For the arrangement of the pressure connections and of the piezo- 

 meter, reference may be made to romm. N°. 97^/. (Plate I.) 



c. Cryostat. 



The cryostat contained, besides the small reservoir /i' two platinum 

 resistance thermometers IT^ for reading the temperature, a stirrer B 

 for the cryostat liqnid (in this case ethylene), and the usual auxiliary 



M H. Kamerlingh Onnes, G. Dorsman and G. Holst. Proc. XVII (2) pg. 'JjO. 

 (Dec. 1904). Leiden. Gomm. N". 14.")&. 



2) These Proc. IX p. 754 (Dec. 1906;. Leiden Gomm. N». 97«. 



