( ^01 ) 



The luimLiers given differ slightly from those communicated before i) 

 on account of a general revision of the calculations. 



The law of mixtures may further be applied, as has been done 

 before ^), to the mixtures of oxygen and nitrogen and the observed 

 rotatory constants may be compared witli those computed from the 

 components. This comparison will prove very satisfactory in the 

 case of air, but less so in the case of the two other mixtures. 



In fig. 12 curves are given showing the relation between 

 magnetic rotatory constant and wave-length, which are drawn by 

 making use of the values calculated from some of the communicated 

 formulae for interpolation. They prove clearly that the dispersion 

 of the magnetic rotation for all gases, oxygen excepted, is pretty 

 much the same. If we consider the large coefficient of magnetisa- 

 tion of oxygen, it becomes evident ^) that the magnetic rotatory 

 dispersion depends on the magnetic properties of the substance, a 

 fact to which H. Becquerel has drawn the attention *). 



It is a striking fact that the order of the gases examined, when 

 they are arranged according to the values of the coefficients di and 

 rfn, should be the same as the one found for the coefficients of 

 magnetisation. This appears from the annexed table : 



') Verslag Kou. Akad. 1896/97, p. 132. Comm. Pliys. Lab. Leiden N». 31, p. 56. 



2) Verslag Kon. Akad. 1895/96, p. 301; 1896/97, p. 132; Comm. Phys. Lab. Ley- 

 den, No. 24 p. 10, N". 31 p. 6. 



'■') H. Becqdeeel. Ann. de Cli. et de Ph. (5) 12 p. 85. Comp. also Verslag Kou. 

 Akad. 189.3/91: p. 31 ; Comm. I'hys. Lab. N". 7 p. 9. 



■■) The extraordinary large dispersion, observed by Becquerkl in substances with 

 negative magnetic rotation drew my attention to the negative constant which Quincke 

 [Uied. Ann. 24. p. 615 (1SS5)] found in amber. A fresh measurement with a piece 

 of transparent, light brown amber of 1.78 cm. thickness actually showed negative 

 rotation. Yet it appeared that the emerging ray of light, even without the action of 

 the electro-magnet, was strongly elliptically polarized. In connection with the theore- 

 tical investigations on the magnetic rotation in doubly refracting bodies (Goüv, 



