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Physics. — “Measurements on the magnetic rotation of the plane 
of polarisation in liquefied gases under atmospheric pressure. 
Il. Measurements with methylehloride? by Dr. L. H. Siertsema. 
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(Communication n°. 80 from the Physical Laboratory at Leiden, 
by Prof. H. KaAMERLINGH ONNES). 
(Communicated in the meeting of June 28, 1902). 
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I. In a previous communication n°. 577) an apparatus for the 
measurement of the magnetic rotation in liquefied gases under atmos- 
pherie pressure was described and a few results with methyl chloride 
were given. Further measurements with this apparatus have not 
fulfilled my expectations, so that it appeared to be necessary to make 
considerable modifications. 
In the first place it was difficult to insulate the apparatus properly 
from heat. It had been packed in cotton-wool, yet if was not easy 
to obtain a perfectly quiet liquid, entirely free from rising bubbles. 
The pressure of as many as six glass-plates between the nicols was 
also very disturbing, owing to thei depolarising influence. This was 
more noticeable after filling the apparatus when tensions appeared 
to arise in the plates, in contact with the eold liquid, which often 
rendered adjustment quite impossible. 
To remedy this defect the nicols were immersed in the cold 
liquid in the tube marked / of the plate in the previous commu- 
nieation. The nicols were lying loose in this tube and were connected 
by a brass wire, running along the outside of the tube. One of 
the nicols could revolve in its holder and could be adjusted at a 
given angle before the apparatus was closed. The rotation for different 
wave-lengths could then be found by measuring the intensity of the 
current required to bring the dark band in the speetrum to that 
wave-length. The apparatus being arranged in this manner some 
measurements could be made with it, but always after some time 
the nicols appeared to have lost their transparency, either because 
the layer of canada-balsam was dissolved or became laminated. 
Nieols with a layer of linseed oil instead of canada-balsam lasted 
longer, and the layer seems not to dissolve so easily, but in the 
long run these too lose their transparency, perhaps in consequence 
of irregular deformation of the calespar by the sudden and intense 
cold of the liquid gas. 
2. Then a new apparatus was constructed, in which also the 
1) Proceedings Royal Acad. of Sciences. May 1900, 
