Rotatory Polarization of Liquids. 395 



furnished with a telescope, to which the eye is applied to receive 

 the polarized ray, and with a divided circle which moves with 

 the analyzing-prism and enables one to appreciate to V the angle 

 through which the prism is turned. The substance to be operated 

 on is placed between the two poles in such a manner as to be in 

 the path of the polarized ray and, consequently, traversed by it. 

 We then determine the azimuth of the angle which gives the 

 passage-tint when the electromagnet is magnetized by passing 

 an electric current through the wires which encircle it. After- 

 wards, by means of a commutator, the direction of the current is 

 reversed, and a second passage-tint is obtained, distant from the 

 first by a certain angle, the azimuth of which is likewise deter- 

 mined. The angle through which the analyzing-prism had to 

 be turned in order to pass from the one passage-tint to the other 

 represents double the rotation of the plane of polarization. This 

 is the angle I have always measured, and which, for the sake of 

 brevity, I call the angle of rotation. After a little practice the 

 operation is effected very quickly and accurately : one eye re- 

 ceives the polarized ray, while, by means of a lens, the gradua- 

 tion of the moveable circle is observed with the other. I will 

 not dwell on other details, easy to be understood, and will merely 

 add that the source of light I used was a gas-burner which gave 

 a very brilliant white light. 



The liquids on which I operate are contained in tubes herme- 

 tically closed at their two ends with glass, which makes it neces- 

 sary to take account of the influence of the glass upon the rota- 

 tion — an influence the existence of which I ascertained by ope- 

 rating on empty tubes, and the amount of which varies with the 

 nature and the thickness of the glass. In order to dispense with 

 this correction I placed the tubes between the interpolar sur- 

 faces so that their two extremities entered to about a centimetre 

 within the cylinders of soft iron, the internal diameter of which 

 is greater than the external diameter of the tubes. Theory indi- 

 cates, and experiment fully confirms, that the portion of the 

 tubes (and consequently their glass ends) which is placed in the 

 interior of the iron of the electromagnet is subject to no influence 

 at all from the magnetism developed by the electric current, and 

 that the magnetism affects only that portion of the tube which 

 may be called interpolar — consequently only the liquid column 

 whose length is equal to the distance of the two polar surfaces*. 



* I have made a great number of experiments to confirm the accuracy 

 of this principle. Thus I placed in the interior of one of the cylinders of 

 the electromagnet some tubes, from 10 to 15 centims. in length, filled with 

 sulphide of carbon ; and never was any effect produced by them upon the 

 polarized ray which traversed that liquid, even when the magnetization was 

 very strong; but when one of the extremities was withdrawn only a few 

 millimetres, an effect was produced. 



2D2 



