( BOO ) 
These are questions which, considering the possibility of obtaining 
important contributions to the knowledge of the influence of density 
upon susceptibility by high pressures in the region where the gaseous 
state of aggregation changes continuously into the liquid make it a 
very attractive work to realise, even from a purely experimental 
point of view, the representation to which we have just referred. 
The work was commenced though only when Prof. Weiss ex¬ 
tended his magnetical researches to very low temperatures and the 
measurements on the magnetization of ferro-magnetic and cognate 
substances at very low temperatures, which were communicated to 
the February Meeting 1 ), were undertaken. With that investigation 
which was carried out at the same time, the present one is very 
closely related, and for part of them we made use of the same 
appliances. In our present investigation we have also in various 
ways made use of Prof. Weiss’s method* *) of determining the magne¬ 
tization by means of the maximum couple exerted by a magnetic 
field of variable direction upon an ellipsoid of the experimental 
substance, a method which had been entirely successful in the other 
research. We must also express the great advantage we derived 
from the continued support given us by Prof. Weiss, and we take 
this opportunity of gratefully acknowledging our indebtedness to him. 
The change with temperature of the specific susceptibility of oxygen, 
the investigation of which was our first object, is of particular 
importance seeing that Curie’s law follows from Langevin’s kinetic 
theory of magnetism*). It was soon apparent to us that this law 
was not valid for oxygen, as was thought, down to —183° C., but 
that it would have to be replaced by another. According to the 
important paper of du Bois and Honda communicated to the January 
Meeting — our experiments had already been completed at that 
time — various elements were found for which Curie’s law did not 
hold at temperatures above 0° C. This at once increases the impor¬ 
tance of the further investigation of oxygen, for which over a definite 
region of temperature Curie’s law is valid, while over another region 
it obeys a second law, viz.: that of inverse 'proportionality to the, 
square root of the absolute temperature. The results concerning this 
law and also concerning the probability of a sudden change in the 
value of the specific susceptibility on solidification will be discussed 
in § 5. 
b P. Weiss and H. Kamerlingh Onnes. These Proc. Jan./Febr. 1910. Comm. fr. 
the Leyden labor, no. 114 (1910). 
*) P. Wass. Jonra. de phys. 4e serie t. VI, p. 661; 1907. 
* •) Lahgevr. Ann. clrim. phys. (8) 5, p. 70; 1905. 
