( 184 ) 



For the assistance rendered me in the different control experiments, 



1 gladly express my thanks to Dr. W. H. Keesom and Mr. H. 

 Filippo Jzn. 



§ 6. Properties of the helium. By the side of important points of 

 difference the properties of' helium present striking points of resem- 

 blance with the image which Dewar drew in his presidential address 

 in 1902 on the strength of different suppositions. 



We mentioned already the exceedingly slight capillarity. 



For the boiling-point we found 4°. 3 on the helium thermometer of 

 constant volume at 1 atm. pressure at about 20°K. This temperature is still 

 to be corrected to the absolute scale by the ail of the equation of 

 state of helium. The correction may amount to some tenths of degrees 

 if a increases at lower temperatures, so that the boiling-point may 

 perhaps be rounded off to 4°. 5 K. 



The triple-point pressure if it exists lies undoubtedly below I cm., 

 perhaps also below 7 mm. According to the law of corresponding states 

 the temperature can be estimated at about 3° K at this pressure. The 

 viscosity of the liquid is still very slight at this temperature. If the 

 helium should behave like pentane, we could descend to below 1,5° K 

 before it became viscous, and still lower near 1° K before it became 

 solid. How large the region of low temperatures (and high vacua) 

 is that has now been opened, is, however, still to be investigated. 



Liquid helium has a very slight density, viz. 0.15. This is smaller 

 than was assumed and gives also a considerably higher value of b 

 than can be derived from the isotherms at — 252°.72 and — 258°. 82 

 now that the points mentioned in § 1 have been determined, viz. 

 about 0.0007 provisionally. The value of b which follows from the 

 liquid state is about double the value of b which was expected 

 [viz. 0,0005], and was assumed in the calculations of Dr. Keesom 

 and myself on mixtures of helium and hydrogen (Oomp. Suppl. 

 N°. lb Sept. '07 § 1 fn. 5). 



From the high value of b follows immediately a small value of 

 the critical pressure, whicli probably lies in the neighbourhood of 



2 or 3 atms., and is exceedingly low in comparison with that for 

 other substances. So when helium is subjected to the highest pres 

 sures possible, the "reduced" pressures become much higher than 

 are to be realized for any other substance. What may be obtained 

 in this respect, by exerting a pressure of 5000 atms. on helium 

 exceeds what would be reached when we could subject carbonic 

 acid e.g. to a pressure of more than 100.000 atms. 



The ratio of the density of the vapour and that of the liquid is 



