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liquid, without our observing the increase of the liquid. And the 

 first time the appearance of the liquid had really escaped our 

 observation. Perhaps the observation of the liquid surface, which is 

 difficult for the first time under any circumstance, had become the 

 more difficult as it had hidden at the thermometer reservoir. However 

 this may be, later on we clearly saw the liquid level get hollow 

 by the blowing- of the gas from the valve and rise in consequence 

 of influx of liquid on applying accelerated expansion, which even con- 

 tinued when the pressure descended to 8 atms. So there was no doubt 

 left that the critical pressure lies also above one atmosphere. If 

 it had been below it, the apparatus might all at once have been 

 entirely filled with liquid compressed above the critical pressure, 

 and only with decrease of pressure a meniscus would have appeared 

 somewhere in the liquid layer ; this has not taken place now. 



The surface of the liquid was soon made clearly visible by 

 reflection of light from below, and that unmistakably because it was 

 clearly pierced by the two wires of the thermoelement. 



This was at 7.30 p. m. When the surface had once been seen, 

 it was no more lost sight of. It stood out sharply defined like the 

 edge of a knife against the glass wall. Prof. Kuenen, who arrived 

 at this moment, was at once struck with the fact that the liquid 

 looked as if it was almost at its critical temperature. The peculiar 

 appearance of the helium may really be best compared with that of 

 a meniscus of carbonic acid e. g. in a Cagniard de la Tour-tube. 

 Here, however, the tube was 5 cm. wide The three liquid levels 

 in the vacuum glasses being visible at the same time, they could 

 easily be compared; the difference of the hydrogen and the helium 

 was very striking. 



When the surface of the liquid had fallen so far that 60 cm*, of 

 liquid helium still remained — so considerably more had been drawn 

 off — the gas in the gasholder was exhausted, and then the gas 

 which was formed from this quantity of liquid was again separately 

 collected. In the course of the experiment the purity of this gas 

 was determined by means of a determination of the density (2,01), 

 which was afterwards confirmed by an explosion experiment with 

 oxyhydrogcn gas added, and further by a careful spectroscopical 

 investigation. 



At 8.30 the liquid was evaporated to about 10 cm 3 ., after which 

 we investigated whether the helium became solid when it evaporated 

 under decreased pressure. This was not the case, not even when the 

 pressure was decreased to 2,3 cm. A sufficient connection could not be 



