ORIGIN OF ELEMENTx\.RY SUBSTANCES. 133 



cerium^ as this element and yttrium are nearly always 

 found associated in the mineral species cerite and yttroce- 

 rite. Moreover^ it will be observed that ,2?= 69 is just 1*5, 

 or 0.75 the atomic weight of cerium, according as it is 

 regarded as 46 or 92. Mendeleeff and other chemists have 

 already proposed 138 as the atomic weight of cerium^, 

 which is double that of x = 6(). MM. Hildebrand and 

 Norton have recently obtained cerium, lanthanum, and 

 didymium in a massive state, and have thereby been able 

 to investigate some of the physical properties of these rare 

 metals f. According to these experimenters the specific 

 gravities of Ce, La, and Di, range between 6 and 6-7. 

 Bearing in mind that elements of approximately the same 

 atomic weights and specific gravities generally belong to 

 different series, and that the specific gravities of analo- 

 gous members in each series increase in the order of their 

 atomic weights, it would appear that cerium does not 

 belong to the same series as lanthanum and didymium. 

 Moreover, consid-fering the important position which x=6() 

 occupies in relation to its analogues Al, Yt, and the posi- 

 tion which these three elements occupy in relation to 

 their homologues Mg, Zn, Cd, and Na, Cu, and Ag, it may 

 be doubted if w = 6() should, up to the present time, have 

 remained undiscovered, especially as all its analogues of 

 the series Th, E, Yt, and Al, are well known. If, there- 

 fore, a? = 69 be cerium, the only element missing in the 

 series H3W is 57 = 42, the analogue of Ga, In, and Tl. As 

 these elements have been discovered by spectrum analysis, 

 it is probable that a:=^2 will also be found by the same 

 means. It may, however, be observed, that the character- 

 istic lines of the alkaline metals in the series Jin, and of 

 their homologues H^n, advance in the blue or violet end of 



* Ann. Chem. Pharm. Suppl. viii. pp. 1 85- 190. 

 t Chem. Soc. Journal, 1876, vol. ii. p. 276. 



