Physics. — "The inaynetlc rotation of the polarisation plane in 

 titanium tetrachloride." I. By Prof. L. H. Siertsema. (Com- 

 municated by Prof. H. A. Lorentz). 



(Communicated in tlie meeting of March 27, 1915). 



Among the substances of which the magnetic rotation of the plane 

 of polarisation has been observed, titanium tetrachloride occupies a 

 particular place, first of all because it is the only diamagnetic sub- 

 Stance known that presents negative magnetic rotation, and then 

 because it is unique in being a liquid, for which the negative 

 magnetic rotation can be observed without the interfering influence 

 of a solvent, and which is tree from absorption bands almost over 

 the entire visible spectrum. Only in the extreme violet, according to 

 my observation at A = 0.420 f*, an absorption limit is found. 



The diamagnetic charactei' has been ascertained by Verdet ') and 

 by H. Becquerel ■') ; observations on magnetic rotation have been 

 made by Verdet "), who only shows tliat the I'otation is negative, 

 and about of the same amount as the positive rotation of water, 

 and by H. Becquerel *), who determines the magnetic rotation for 

 six Fraunhoeer lines. It appears from these measurements that the 

 rotations are in inverse ratio to the fourth power of the wavelength, 

 hence they follow an entii-ely different law from that found for the 

 positive rotation. 



An attempt to ascertain whether the later dispersion formulae 

 derived from the theory of electrons can be applied to this substance 

 made me realize the desirability of extending the material of obser- 

 vation by the carrying out of new measurements. 



The measurements have been carried out by a penumbra method 

 and spectral analysis. 



The liquid was in a glass tube of a length of 265 mm. and a 

 diameter of 25 mm., closed by plane-parallel glass plates 1 mm. 

 thick. This tube was placed in a coil of wire 182 mm. long, so 

 that the closing plates were a few centimetres outside the coil. 



It appeared from a measurement with an empty test tube that 

 the magnetic rotation in the glass plates was imperceptible. The 

 coil contained 3186 windings; between the windings and the coil 



1) E. Verdet, Ann. de Gh. et de Ph. (3) 52 p. 156 (1858). 



2) H. Becouerel, Ann. de Ch. el de Ph. (5) 12 p. 63 (1877). 

 S) E. Verdet, loc.cit. 



♦) H. Becquerel, Ann. de Ch. el de Ph. (5) 12 p. 35 (1877); G.R. 85 p. 1229 



'1877). 



