324 
the specimen used by KaAMERIINGH Onnes and Perrier). For this not 
quite anhydrous ferrous-sulphate III we found the data given in table II 
(p. 323). 
§ 3. Deviations from Curw's law. In previous Communications 
an attempt was made to establish a law other than Curim’s (which 
from Table I does not hold for anhydrous ferrous sulphate) to 
- represent empirically the variation of 4 with temperature; for this was 
given the law ¥V 7’=const., which did quite well indeed represent 
the various observations then under consideration. The analogy of 
phenomena exhibited by ferric sulphate, which lead one to believe 
that this substance exhibits ferromagnetism at low temperatures, 
suggested to us to express y—! as a function of the temperature, 
and we found that the formula x(7-+A') = C’, which has also been 
used by Weiss and Foëx*)*) was worth trying with positive values 
of A' and C”’. As long as we keep above — 208°C. this formula is 
quite satisfactory for the representation of the deviations from Curtr’s 
law at low temperatures found by KAMERLINGH ONNEs and PERRIER 
and by us up to the present; we shall give several instances of this 
in § 7. The variation of y as a function of the temperature can then 
be expressed for ferrous sulphates of different degrees of dryness by 
ascribing different values to A’. Below the maximum gt remains 
still linear, at least to a first approximation, but the constant, C", 
which replaces the Cvriw constant in that region, is negative, as is 
also A", the constant which replaces 4’. 
The results obtained for ferrous sulphates I, II, and III are shown 
1) Starting with the idea of corresponding states for para- and ferro-magnetic 
substances, we were led to the formula y (Z7’+ A') =C’ by an attempt to deter- 
mine the absolute temperature © of the possible Curie point with the help of 
experimental data in the suspected region of the “magnétisme sollicité”. We found 
@ negative, which brought to our minds the notion of the inverse field by which 
Vorer has tried to explain certain peculiarities of the ZEEMAN effect as shown by salts 
of the rare earths. It was only after we had represented the deviations from CURIE's 
law shown by paramagnetic substances at low temperatures by means of this 
molecular diamagnetic field that we noticed that Werss and Foëx had in the same 
way represented the behaviour of ;-iron and the nickel alloys above the GURIE 
point. Werss and Fox show that there is no prima facie cause why the Werss 
molecular field could not occur with the opposite sign. It speaks well for the 
reasonableness of the hypothesis that we should be led to it for entirely different 
EL 
substances and under circumstances in which the quantity 7 fundamental in 
paramagnelism, is so much greater than in the experiments made by Werss and Foéx. 
2) After this communication was printed in dutch, we received the dissertation 
of A. Preuss, Ziirich 1912, in which there is found also a negative molecular 
field for the alloys of Fe with less than 16°/) Go. [Note added in the translation. | 
