( 654 ) 
An analogy thus seems to exist between this and compression and 
expansion by heat, for which van der Waals’s theory and law of 
corresponding states are supported better as a rule in the neighbour-! 
hood of the critical point than at low reduced temperatures where 
the ideal representations of the molecule and of molecular attraction 
no longer cover the phenomena sufficiently well and the differences! 
between the specific properties of the real molecules appear. 
The hypothesis that molecular magnets are essentially invariable 
would be established conclusively if there existed simple relations 
between the magnetic moments as calculated per atom, which one 
might be led to suspect from the increase by regular steps of the 
saturation-magnetization of the three metals. 
The following table in which the numbers in the first column are 
taken from the paper 1 ) referred to above and in which the relative 
increase for cobalt is estimated from comparison with iron and nickel 
shows that this is not the case. The data are not corrected for the 
dilation (see note 2 on p. 11 ). 
Specific Increase by Specific j Atomic Moment 
saturation at reduction to saturation at 1 weight or ' of 
temp. ( ). low temp. ! low temp, j '/j mol. wt. i gram-atom. 
54.6 (17° C.) 
162 (17° C.) 
217 (20* C.) 
, 90.75 (150.8 C.) 
In connection with this we must not lose sight of the fact that 
although the proof that the above magnitude is of great significance 
may have escaped us, still there is nothing whatever to justify an 
opposite conclusion. 
When we look upon our measurements as a whole we remain 
inclined to retain the hypothesis that in ferromagnetic substances the 
magnet,c atom does not in itself change much with temperature. 
There were mdeed reasons for questioning if this approximate inva- 
r,ability, granting that it was proved in other circumstances, still 
existed at extremely low temperatures. Electrical resistance of 
rnetaJs, phosphorescence of snlphnr compounds, absorption of light 
by the salts of the rare earths with or without magnetic field, 
all, at very low temperatures, exhibit characteristics that one may 
>) P. Weiss. Arch, des Sc. phys. et nat. and Journ. dc Phys. 1910. 
