( 585 ) 



The ])henoinenon was very clearly visible, and the amount of the 

 separation is evidently pretty large. Chietly in consequence of rime 

 ojily a short observation was possible, which frustrated photographing 

 with long exposition. Our 'j^ plate proving unfit, we could not yet 

 determine the circular polarisation ; nor did our arrangements allow 

 the observation of a transversal eifect for equatorial direction of rays ; 

 we hope to be able to remove these difficulties in course of time. 



§ 8. Of tlie other metals of the third series there exist among 

 others also the following compounds with cliaracteristic absorption 

 spectra^): Sapphire, C0CI3, KMn O^, Fe CI3, the last mentioned with 

 strongly negative rotation, and some compound rhodanides. Of these 

 we examined : 



Cobah-ainmoniaii). rliodanlde [(NH J, (Jo (CNS)J in rather diluted 

 alcoholic solution: at 18° dimly outlined band 594 — 663; limit of 

 absorption at 696. At — 193° the blue solution became solid, but 

 remained transparent, though of a paler colour ; the bands become 

 narrower as the temperature falls ; at last vague bands are seen 

 580—586, 597—605, and 618—620; shade up to 645. 



§ 9. Fifth series. This now includes, arranged according to incre- 

 asing atomic weights from 140 to 175: Cerium, Pmseodymiuui, 

 Ni'odymium, Samarium , [Europium]. Gadolinium, [Terbium, Dyspro- 

 siiwi], Erhiiim, Ytterbium"-); the three placed between brackets 

 were not yet obtainable in 1899; the compounds of the others then 

 proved strongly paramagnetic, with a maximum for erbium. The 

 visible absorption spectra of the metal compounds printed in italics 

 exhibit the well-known highly selective properties ; possibly an accu- 

 rate investigation would yield a similar result also for the "white" 

 compounds of Ce, Gd and Yb, perhaps only in the ultra-violet or the 

 infra-red spectrum. We have confined ourselves for the present to 

 compounds of neodyinium and erbium, which had been used for 

 the magnetic measurements in 1899. We wish to express our 

 indebtedness to Prof. Rosenheim and Dr. R. J. Meijer for their kind 

 assistance in this special chemical department. 



^) Cf. J. M. HiEBENDAAL, thesis for the doctorate, Utrecht 1873. 



~) Gf. R. J. Meijer, Handb. der anorg. Chemie 3 p. 129—338, Leipzig 1906. 

 Scandium Yttrium and Lanthanum have lower atomic weights and diamagnetic 

 compounds. Holmium ami Tliulium have not yet been sufficiently chemicrlly deter- 

 mined. Further the following investigations have lately been published on the 

 absorption spectra : \V. Rkcu Zfitsclir. wiss. Photogiv 3 p. 411, 1906; — Helen 

 Schaeffer, Physik. Zeitschr 7 p. 822, 1906; B. Schaeffers. Dissert. Bonn 1907. 



40* 



