208 



^ 5. The rotation-dispersion of these optically-active iridium-seilts 

 was measured in the same way as formerly by means of a great 

 polarimeter of Schmidt and Haensch, with threefold field, and equipped 

 with a monochromator. The tube was always 20 cm. long, and 

 determinations were made for a whole series of wave-lengths. Even 

 in thin layers of the liquid the spectral region of the transmitted 

 light appeared to be appreciably limited by absorption, also in the 

 case of not very concentrated solutions. 



Waves larger than 6850 A.U. were never transmitted to a sufficient 

 degree, while even in a solution of J 7o "O exact determinations 

 could be made for wave-lengths smaller than 5300 A.U. To investigate 

 the shape of the dispersion-curve also for shorter wavelengths, it 

 was therefore necessary to use very dilute solutions, of 0,2 "/„ and 

 0,1 7o or less. We have used four such solutions for this purpose, 

 containing respectively one grammolecule of the hydrated salt in 

 14,57 Liters {a) of the solution, in 57,73 Liters (b), in 228,86 Liters (c), 

 and in 413,7 Liters (d). 



In the following table the mean values are taken into account, and 

 the molecular rotations calculated from them; in fig. 2 these results 

 are moreover graphically ülotted, in their relation to the light used: 



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The values obtained with the different solutions agreed very well 



