321 



final drop in the magnetization was found by them to lie between 

 200" and 240° for fields of about 10 gauss. First of all was avail- 

 able the sample I, which had been etched with H^SOg. B'ig. 1 

 repj-esents the temperature curve between — 78° and + 220°. The 

 curve extended cuts the axis of abscissae near 235° in agreement 

 with the results of the other obser\ ers just cited. 



I received from Dr. Hilpert, also, two ditferent pulverized cemen- 

 tites which after pressing in the dry state could be polished fairly 

 well. These, it should be noticed, gave entirely distinct dispersion 

 curves, ditferent from those for sample 1, which moreover differed 

 among themselves. While the latter shows a maximum and minimum 

 of the rotation respectively in the violet-blue and in the green-yellow, 

 these singularities are displaced in the case of samples II and III. 

 All, however, gave strong negative rotations which will now be 

 discussed more in detail. 



Iron Carbide II (4,8 7o C, about 20 7o li'ee iron, Hilpert) shows 

 a powerful rotation (between — 18' and — 21') with a numerical 

 maximum in the yellow and a minimum in the orange-red. (Fig. 2). 

 The mirror, which had a distinct yellowish appearance, scarcely 



21 



Proceedings Pioyal Acad. Amslerdam. Vol. XVl. 



