( 605 ) 



Index. 

 1,600 



J,75() 



In tliis vsla^ tlio Zi»() [)lavs the part of llie M.i'O. Wlioii ZiiO is 

 rej)laced hv Mg<), llie index remains llie same. 



Finally tiie index of tlie following' minerals was delei-inined -. 

 Quartz SiO, 100 "/„ 1,475 



Chrysolite „ 40-45 1,610 



Orthoclase „ 65 1,485 



The last mineral, the pure K.x\l. silicate eonsecpienlly does nol lit 

 into the composed series. After mixing with some grains Fe./)3 

 (5 — lO^/"} and fiisijig anew the index was ]-aised to 1,510. 



The method described above can be of [)raclical use for a ([nick 

 determination of the Si()j percentage of slags from the i-etVactixe 

 index with an accurac}^ of ± 2 "/„. 



A word of thanks foi- ihe aid and advice to ihe Professors Dr. J. L. ('. 

 ScHROEDE« VAN DKR KoT,K and 8. .1. \'khmai;s .li". nuiv find n jjlacc here. 



Mineralogy". — ''()n an " FAscurose' of the S(. Gottli'ir(J.'\ V^y 

 G. B. HoGENRAAD. (Communicated by Pi-of. J. li. C. Schrohder 

 VAN DER Kolk). 



Some time iigo 1 tried to get a Hematite-sti-eak with a so-called 

 "Eisenrose." I did not succeed however, for to my astonishment 

 the streak was not red but black. Se\ei-al explanations came to 

 my mind : 



1". that the jnineral was somewhat friable, which was the cause 

 that Ihe streak couhl not consist of the very finest i)articles. Hut in 

 rubbing the black colour remained; only the outlines showed a 

 reddish-brown tint. The same was slated with about 25 other pieces 

 of the sajne finding-place. So that tiie explanation j)rov('<| to be not 

 the right one. 



