311) 



NaHCOj and CaCl.^ ^). A few measurements were made at a low 

 temperatui-o. \\\c whole system being' imbedded in carbon <liüxide 

 snow. 



Otiierwise the experimental arrangement was the same as hitherto *) 

 except that instead of the small half-ring a large du Bois fnJl-ring 

 electromagnet was used. With this all of the mirrors investigated 

 were capable of saturation, since the strongest field was estimated 

 at from 30 to 40 kiloganss. 



The direct-vision high-luminosity monochromator ') was recalibrated 

 at several points with the aid of a mercurv vapour lamp. The obser- 

 vations proper were made exclusively with au arc lamp. The field 

 was reversed each time to eliminate constant errors and the observed 

 values corrected for the FAKADAY-rotation in the air-path. In ^ iew 

 of the smallness of the rotations it proved still impossible to determine 

 the ellipticity. 



As regards the material, it was found that most pulverized sub- 

 stances by moderate compression with or without binding material 

 could also be worked up into pastils which were more or less 

 capable of polish. 



The present research aims principally at a general knowledge 

 with regard to the magneto-optical properties. Especially is this true 

 of the temperature curves the range and precision of which need 

 still to be extended considerably. 



In the following tables 1 denote as hitherto by. X, the wave length 

 of the light in question in n{^i ; A, the double rotation on reversing 

 the current as read off the scale in mm.; =t ds, the mean error 

 respectively in minutes or in percent. ^V is the number of readings 

 made for each direction of the current, which varied according to 

 the brightness of the mirror from 10 to 30 ; t, the temperature ; 

 0^, the temperature at which the magnetizability vanishes. 



To Geh. Rat. G. Tammann, Göttingen, Prof. P. Weiss, Zurich, 

 Privatdocent Dr. S. Hilpert, Charlottenburg, and to the firm, 

 Griesheim-Elektron, I am greatly indebted for kindly supplying 

 valuable materials. 



Binary Manganese compounds. 

 Manganese Arsenide (Mn As. Hilpert, &^ = 45°). The rotation in 



1) R. SiEDENTOPF Dissert p 8 Göttingen 1S97. 



S) Cf. St. Loria. These Proceedings 12. p. 835. 1910; 14, p. 970, 1912. 



3) H. DL' Bois, Ztscbr. füi- lustr. Kunde 31, p. 1. 1911. 



