204 



some hours at high pressure, (al)ü\e 1500 attii.), it appeared to be 

 unfit for accurate measurements af'ler that time. When the apparatus 

 was opened, the mercury appeared to be quite contaminated, the 

 glass tube and the platinum contacts being also covered by a black 

 substance. 



Though in view of Amagat's experiments it could hardly be 

 supposed that this substance was mercury oxide, formed by the 

 action of the oxygen on the mercury, experiments of various kinds 

 made it impossible to assume another cause. The supposition that at 

 high pressure amalgamation of the platinum took place, proved 

 eiToneous, for in the black substance no trace of another metal than 

 mercury could be demonstrated. Also the humidity of the air proved 

 to be entirely without influence. When it finally appeared that neither 

 filling with hydrogen nor with nitrogen yielded any trace, we 

 could not but conclude that we had really to do here with the 

 same phenomenon that Kuenen and Robson^) and Keesom^) had 

 observed when using closed air-manometers, namely that oxygen and 

 mercury act on each other at pressures of about 100 atm. Keesom, 

 however, describes a slow action, which only after the lapse of 

 months manifests itself (dearly ; whereas we could demonstrate the 

 formation of mercury oxide with certainty already after a few hours 

 on account of the so much higher pressures. 



How it is that neither in his determination of air-isotherms nor 

 in that of oxygen Amagat was troubled by this action, we cannot 

 explain. After we had once ascertained it, the use of oxygen and 

 oxygen mixtures was of course excluded. We therefore resolved to 

 begin with measurements of hydrogen, which is most easily obtained 

 in very pure state. The choice of tiie temperature of our measure- 

 ments was directed by the desire to obtain a direct comparison with 

 Amagat's measurements on one side, and a supplement to Schalkwuk's 

 very accurate measurements at low pressures on the other side. 



§ 2. Filling of the apparatus loitli pure ky drogen. 



Most of our determinations have been made with hydrogen from 

 the factory "Electro" at Amsterdam, which sells cylinders of com- 

 pressed electrolytically prepai'ed hydrogen. For the further purifi- 

 cation and the filling of tlie apparatus with purified gas the arran- 

 gement was used of which fig. 12 gives a schematic representation. 

 It fits on to the most left hand part of fig. 6 at /. 



1) Phil. Mag. Jan. 1902, p. 150. 



2) Diss. p. 50—53. Thesis III. 



