284 



this inetliofl tlie problem of |)iramagnetic saturation whicli is also 

 fiiiidamental to Wkiss's theory of ferroinagnetism. In the first phace 

 it was iiecessarj to liave a substance which might be expected to 

 obey Curie's law, which also tbllows from Langkvfn's theory, down 

 to heliiim-temperatnres ; in the second place the substance must have 

 a high value of a,n. Both properties I hoped to find combined in 

 crystallized gadolinium-sulphate, a qnantity of which Professor Urbain 

 some time ago had very kindly put at my disposal. 



Earlier investigations in conjunction with Perrier and Oosterhuis had 

 shown, (hat gadolinium-sulphate follows Curie's law down to the free- 

 zing point of hydrogen and does not show any sign of saturation, which 

 as Langevin's theory shows, if it existed at that temperature, would 

 be ferromagnetic in its nature, as paramagnetic saturation at the 

 value of a which could be reached would not yet be clearly obser- 

 vable. The number of magnetons calculated according to Weiss is 

 large (38). That gadolinium-sidphate would still obey Curie's law at 

 helium-temperatures I felt justified to infer from the fact, that it is 

 a "diluted" paramagnetic substance. The gadolinium-atoms, separated 

 as the}^ are e.g. by the water of crystallisation, are at great distances 

 from each other, and this Dr. Oosterhuis and I in Comm. N°. 1396' 

 found a favourable circumstance to Curie's law being valid down to 

 very low temperatures. 



The experiments have given a confirmation of Langevin's theory 

 which is at least cjualitatively even now complete. Before an opinion 

 can 1)6 formed as to the cpiantitative agreement various corrections 

 will first have to be investigated. The most important of these which 

 must not be neglected, especially when the validity of Curie's law 

 is to be tested, is the demagnetising action of the paramagnetisation 

 itself, as the latter attains exceptionally high values. As an instance 

 I may mention that with 0,345 gram of gadolinium-sulphate at 2° K. 

 in a field of 15 kilogauss there was observed an attraction amounting 

 to over 100 grammes. Another circumstance that one should keep 

 in mind is that the object of observation consists of small crystals 

 packed on each other. 



The measurements consisted in determining the attraction in a 

 non -homogeneous field, the gadolinium-sulphate in the cryostat being 

 cooled first in liquid hydrogen under normal pressure, next in hydrogen 

 under reduced pressure, next in helium boiliiig at ordinary pressure 

 and finally in helium under 4 mms, the apparatus and the fields 

 being the same each time. 



The measurements at the boiling point of hydrogen (20.^3 K.) 

 had the object to obtain the force at a given point for a given 



