( 446 ) 



tlic (IcIcriiiiiiJilioii of isorlieiTiis, rctai'dcd ihc (leteniiiualioii ol" tlic 

 equation oï sUile of heliimi iiiid ar^oii tor a lon.U' lime after ('omiii. 

 N». 69. 



The investigations on llie isothei-nis of liydroüen are in pi-ogress, 

 and yielded already resuils laid down in conininnieations N". 78, 

 117", 99' and 100', wliich, I hope, will soon he followed hy others, 

 in (he meantime, howexer, also the diHieulty of ohtaining pure helium 

 has been quite, that of oblainin.u' pure argon nearly overcome. The 

 sneeessful pre[iaration of jaire helium was ehiefly due to the hydrogen 

 eireulation (Comm. N". 94/) yielding the required litpiid hydrogen. 

 So the tirst measurements from the series whieh will i-efer to the 

 monatomie gases and Iheir binary mixtures, can already be coin- 

 municated. 



They concern the isotherms of helium, which haxe now taken 

 the |)lace occu[)ied by the isotherms of hydrogen before the hydrogeji 

 was liquelied. Among others the isotherms must lead to the 

 calcidation of the critical (piantities for helium. From the now com- 

 nuinicated determinations of the compressil)ilily along dilferent isotherms 

 at densities which are comi)ai'atively small and differ oidy slightly, 

 the critical temperature can already be calculated by approximation. 



§ 2. Suri:ei/ of the ileteniilnatioiis. 



This investigation conqirises some six detcrnunations of isothei-nis. 

 The temperatures at which they were made, were kept constant and 

 detei-mined in the same way as in the determinations of isotherms 

 for hydrogen [)ul)lished in preceding Communications N". 97" (April 

 J 907), N"ï 99' (Sei)t. 1907), N». 100' (Jan. 1908). T\\2 readings of 

 the hydrogen thermometer were reduced to the absolute scale by 

 means of formula (4) of Comm. N". 97'' (Sept. 1907) with the new 

 coefficients of § 2 of Comm. N". 101''. The six temperatures thus 

 reduced to which the isotherms refer, are : 



+ 10(P.35, 20^00, 0\ ^103^57, — 182\75 and — 216°.56. 



Besides the measurements at the two standard temperatures 0'' C. 

 and 100^ (J. ^) and those at low temperatures a determination was 

 made at 20"^ C. to ol)tain data for the calculation of tlU' (piautity of 

 gas in the stem of the piezometer and in the other parts, which 

 remain at the ordinary temperature during the measurements. 



For all these isotherms the densities, at which the |)ressure was 

 observed, lie about between the same limits which were set by the 



1) The results al (1. ami IOC (1. are iiu:()ni|)atil)lr wilti those of Ramsay and 

 Traveks, wliich, indeed, show strange devialioiis. 



