( 603 ) 



field, wc get the combiii.ilioiis 2.o, l.o, 1.2 for the vibrations 

 normal fo the fieUI. Experiment sliows that the divergences of 

 the pairs of doublets in these three cases are very different. 

 Thus for a bantl of spectrum 1, the vibration being normal to the 

 field, the magnelic doublet is different according as the direction 

 normal lo the field has the principal direction 2 or 3. The pheno- 

 menon is clearly seen in the figure which represents ^^the] group of 

 bands in the orange for neod} niium sulphate at — 259°. Fig. 7 

 PI. V gives a survey of the phenomena of the changes 'with the 

 temperature and the magnetic field in the l)lue of neodymiura sulphate. 

 According to theory it follows from this that each "of the" three diffe- 

 rent directions has a different magnetic constant, and that therefore 

 the vibi'ating system presents three different masses for the three 

 kinds of vibrations. 



As the corresponding bands in the two spectra occupy the 'same 

 or oidy slightly different places, it follows that ^in^first approximation 

 the constant of" the quasi-elastic force in each of^the three directions 

 must l)c proportional to the mass in that direction. 



Physics. — ''On th.e equation of state of a substance in the neigh- 

 bourhood of the critical point Ilqald-c/as. 1. The disturbance 

 function In die neujhboTirhood of the critical state." By 

 Prof. Kamerlinoh Onnes and Dr. W. H. Keesom. Communi- 

 cation N". 104^ from the Physical Laboratory at Leiden. 



§ 1. The great compressibility of a substance in the neighbour- 

 hood of the critical point liquid-gas and the properties connected 

 WMth this, (such as the small variation of the thermodynamical 

 potential at isothermal compression etc.) — which are derived from 

 VAN DEK Waals' original equation of state and better still from^his 

 latest considerations about the compressibility of a molecule ^) — render 

 it necessary that in deriving conclusions from observations in the 

 neighbourhood of tliat condition w^e must take into account various 

 circumstances, otherwise unnecessary for the experimental investiga- 

 tion of the equation of state of a homogeneous substance consisting 

 of one component, which investigation includes that of the quantities 

 of saturation etc. 



It is well-known that owing to the great compressibility the 

 thermodynamic equilibrium is difficult to attain, in fact it has often 



^) Gomp. VAN DER Waals, Proceedings June '03, 



