326 



Table 375. 

 MACNETO-OPTIC ROTATION. 



Faraday discovered that, when a piece of heavy glass is placed in magnetic field and a beam 

 of plane polarized light passed through it in a direction parallel to the lines of magnetic force, 

 the plane of polarization of the beam is rotated. This was subsequently found to be the case 

 with a large number of substances, but the amount of the rotation was found to depend on the 

 kind of matter and its physical condition, and on the strength of the magnetic field and the 

 wave-length of the polarized light. Verdet's experiments agree fairly well with the formula — 



where <: is a constant depending on the substance used, / the length of the path through the 

 substance, H the intensity of the component of the magnetic field in the direction of the path 

 of the beam, r the index of refraction, and A. the wave-length of the light in air. If H be dif- 

 ferent, at different parts of the path, IH'vs, to be taken as the integral of the variation of mag- 

 netic potential between the two ends of the medium. Calling this difference of potential v, we 

 may write Q = Av, where A is constant for the same substance, kept under the same physical 

 conditions, when the one kind of light is used. The constant A has been called " Verdet's con- 

 stant," * and a number of values of it are given in Tables 376-380. For variation with tempera- 

 ture the following formula is given by Bichat : — 



R = Kq (I — 0.00104/ — 0.000014/2)^ 



which has been used to reduce some of the results given in the table to the temperature corre- 

 sponding to a given measured density. For change of wave-length the following approximate 

 formula, given by Verdet and Becquerel, may be used : — 



where ju is index of refraction and A wave-length of light. 



A large number of measurements of what has been called molecular rotation have been made, 

 particularly for organic substances. These numbers are not given in the table, but numbers 

 proportional to molecular rotation may be derived from Verdet's constant by multiplying in the 

 ratio of the molecular weight to the density. The densities and chemical formulae are given in 

 the table. In the case of solutions, it has been usual to assume that the total rotation is simply 

 the algebraic sum of the rotations which would be given by the solvent and dissolved substance, 

 or substances, separately; and hence that determinations of the rotary power of the solvent 

 medium and of the solution enable the rotary power of the dissolved substance to be calculated. 

 Experiments by Quincke and others do not support this view, as very different results are 

 obtained from different degrees of saturation and from different solvent media. No results thus 

 calculated have been given in the table, but the qualitative result, as to the sign of the rotation 

 produced by a salt, may be inferred from the table. For example, if a solution of a salt in water 

 gives Verdet's constant less than 0.0130 at 20° C, Verdet's constant for the salt is negative. 



The table has been for the most part compiled from the experiments of Verdet.t H. Becque- 

 rel,t Quincke, § Koepsel,|t Arons,1[ Kundt,** Jahn,tt Schonrock.tJ Gordon, §§ Rayleigh and 

 Sidgewick,|||| Perkin,1"1[ Bichat.*** 



As a basis for calculation, Verdet's constant for carbon disulphide and the sodium line D has 

 been taken as 0.0420 and for water as 0.0130 at 20° C. 



* The constancy of this quantity has been verified through a wide range of variation of magnetic field by 

 H. E. J. G. Du Bois (Wied. Ann. vol. 33). P- "37. '888. 



t " Ann. de Chim. et de Phys." [3] vol. 52, p. rag, 1858. 



X " Ann. de Chim. et de Phys." [5] vol. 12; " C. R." vols. 90, p. 1407, 1880, and 100, p. 1374, 1885. 



§ " Wied. Ann." vol. 24, p. 606, 1885. 



II " Wied. Ann." vol. 26, p. 456, 1885. 



i " Wied. Ann." vol. 24, p. 161, 1S85. 



** " Wied. Ann." vols. 23, p. 228, 1884, and 27, p. 191, 1886. 

 tt " Wied. Ann." vol. 43, p. 280, 1891. 

 it "Zeits. fiir Phys. Chem." vol. n, p. 7531 1893. 

 §§ "Proc. Roy. Soc." 36, P- 4. "883. 

 Iljj " Phil. Trans. R. S." 176, p. 343, 1885. 

 Itil "Jour. Chem. Soc." 

 **• " Jour, de Phys." vols. 8, p. 204, 1S79, and 9, p. 204 and p. 275, 1880. 



Smithsonian Tables. 



