322 
Physies. — “Magnetic Researches. VI. On paramagnetism at low 
temperatures’. By H. KAMERLINGH Onnes and E. Oostsrauis : 
Communication n°. 129° from the Physical Laboratory at 
Leiden. 
(Communicated in the meeting of June 29, 1912). 
§ 1. Introduction. In the present experiments which form a conti- 
nuation of those discussed by KAMERLINGH ONNES and PERRIER in Comm. 
Nes, 122t and 124° we have again measured the attraction exerted by a 
non-homogeneous field upon a long cylinder of the experimental sub- 
stance. Unless where we state otherwise, the experimental substance 
was finely powdered and contained in a glass tube just as was done 
in the researches referred to. In the present experiments, however, 
we adopted a device which had only been tried a few times in the 
former series, and, in order to eliminate the effect of the glass, the tube 
was taken twice as long as the part of it which contained the 
powder, so that the two halves were the same except that one was 
evacuated and the other held the powder; the evacuated part was 
separated from the other by a plug of cotton wool which was placed 
in our experiments at about the centre of the field of our Wess 
electro-magnet. We now balanced the attraction by gravity, and 
instead of allowing the tube to be drawn down by the attraction 
of the field and to be raised to its zero position electromagnetically, 
the tube was now drawn up by the action of the field and was 
brought down again to its zero position by weights. The modified 
form of the apparatus allowed much greater forces to be measured 
without involving any considerable alteration; we shall return to its 
description whenever a detailed account is given of the apparatus 
used in the former experiments. 
§ 2. Anhydrous Ferrous-sulphate. Comm. No. 124a stated that 
it was intended to investigate this substance at temperatures available 
with liquid nitrogen, so as to fix more definitely the temperature at 
which y attains its maximum value, which lay according to the 
experiments then made between 143° K. and 20° K. While this 
particular investigation was our principal aim, at the same time we 
repeated the measurements previously obtained at other temperatures. 
The salt was dried by heating for some time in vacuo to 280° C. 
special care being taken with this operation. We obtained the 
following results: (see table I, p. 323). 
If we compare these with the results given in Comm. No. 124a 
for ferrous-sulphate which was practically anhydrous we see that a 
small admixture of water diminishes the value of yx, and that to a 
