329 
with A’ and C’ positive. Over the whole of this region it shows 
therefore a disturbance of the first kind, which is to be ascribed 
to the occurrence of a Weiss molecular field of opposite sign. 
Down to —208° C., and perhaps lower, manganese chloride is 
normal. At hydrogen temperatures it deviates in a manner which 
may to a first approximation be represented by x(7’-+ A')=C, or, 
in other words, the disturbance throughout this region is of the first 
kind. Crystallised ferrous sulphate behaves in exactly the same way. 
(Comm. N°. 1222). 
Both anhydrous ferrous sulphate [ and ferrous sulphate not quite 
anhydrous (see $ 3) show a disturbance of the first kind down to 
— 208°C. and probably to about — 250°C.; at hydrogen tempe- 
ratures they show a disturbance of the second kind (both A” and 
C" negative). 
At low temperatures down to — 208°C. anhydrous ferric sulphate 
exhibits the same disturbance of the first kind as anhydrous ferrous 
sulphate. At hydrogen temperatures it exhibits the deviations which 
are discussed in the following section. 
§ 8. Ferric sulphate at hydrogen temperatures. For the first time 
in the course of our observations we here found a dependence of 
the susceptibility upon the magnetic field which leads one to presume 
the existence at these temperatures of ferromagnetism in a substance 
which at ordinary temperatures is paramagnetic. We must in the 
meantime confine ourselves to this general remark. Accurate data 
giving magnetisation as a function of the field at different tempera- 
tures cannot be immediately deduced from the attractive force exerted 
upon a long cylinder in a non-homogeneous field, as long as y remains 
an unknown function of H. The investigation has therefore in the 
first place been continued with a cylinder of short length (a disc) 
of ferric sulphate placed in a certain part of the field at which 
both AH and oil 
Oa 
We must refer to a subsequent paper for the results obtained and 
for the deductions which may be drawn from them. 
are known. 
22 
Proceedings Royal Acad. Amsterdam, Vol. XV, 
