Rotatory Polarisation of Liquids. 399 



nets whose polar surfaces were smaller, the increase of distance 

 of those surfaces would produce a greater diminution of the 

 magnetic force. Besides, this has been verified by M. Verdet 

 in his first memoir on magnetic rotatory polarization*, in which 

 he has established the two important laws we have mentioned, 

 namely : — the rotation of the plane of polarization is proportional 

 to the intensity of the magnetic action ; and it is likewise propor- 

 tional to the thickness of the substance through which the polarized 

 ray passes, the intensity of the magnetic action remaining constant. 

 It seems to me that the phenomenon of magnetic rotatory po- 

 larization might be usefully applied to determine the intensity 

 of the force existing between two magnetic poles, in what Fara- 

 day calls the magnetic field. It would only be necessary to place 

 between the polar surfaces a liquid the coefficient of magnetic 

 rotatory polarization of which is known, taking care that the 

 tube containing it shall be hermetically closed at its two ends 

 with glass disks. Then the angle of rotation with this liquid 

 should be determined in every case in which we wished to know 

 the intensity of the magnetic field f. The ratio between the 

 angles thus obtained would give the ratio between the corre- 

 sponding intensities of the magnetic fields. This procedure is 

 applicable to every form of electromagnet : it would only be ne- 

 cessary to adapt to the electromagnet an apparatus for the pro- 

 duction of magnetic rotatory polarization, which would in no 

 way obstruct any other experiments with the electromagnet — 

 those, for instance, relative to diamagnetism. 



§ 2. Determination of the Specific Magneto-rotatory Power of a 



few Liquids. 



I designate by the name of specific magneto-rotatory power 

 the ratio of the magnetic rotatory power of a body to that of dis- 

 tilled water taken as unity. After various trials, I resolved, as 

 I have said, in all the following experiments (with a few excep- 

 tions, which I will indicate) to give a length of 10 centims. to 



* Ann. de Chim. et de Phys. vol. xli. p. 408 et seqq. 



t It would be necessary to take care always to use the same tube filled 

 with the same liquid, to sarve as a standard, and to determine preliminarily 

 the rotation produced by it upon the polarized ray under the influence of a 

 magnetic force to be taken as unit. The liquid chosen as a standard should 

 be one susceptible of being always reproduced identical. Distilled water 

 would doubtless be the best ; but sulphide of carbon would be preferable 

 for small intensities, on account of its three times as great sensibility, and 

 can likewise be procured quite pure. I think it would be advisable to take 

 as unit the force of the magnetic field corresponding to a rotation of 5° 

 produced by a liquid column of 5 centims. length and with an interpolar 

 distance of 5 centims. It would be easy afterwards to reduce this unit to 

 any other magnetic unit. 



