538 



NATURE 



{April ^, 1889 



manner gallium was discovered, in 1875, by Lecoq de 

 Boisbaudran. In consequence of the sharpness and the 

 well-marked character of these spark spectra, they are 

 relied on by chemists as certain proof of the identity of 

 any two elements which yield identical spectra. 



Next was introduced the systematic study of the ab- 

 sorption spectra seen when a beam of light is passed 

 through certain transparent solids or through solutions of 

 various substances. One of the earliest observers in this 

 branch of spectroscopy was Dr. Gladstone, who, in 1858, 

 read before this Society a paper on the absorption of 



light by various metallic salts, and gave the first descrip- 

 tion of the absorption spectrum of didymium. This 

 branch of spectroscopy has proved not less fruitful in the 

 recognition of new metallic elements. 



In the investigation of the rare earths my principali 

 object has been to separate the true from the undemon- 

 strated and spurious, verifying the true, rejecting the 

 spurious, and reducing as far as possible the number of 

 the doubtful. In the following table I have given a list 

 of the so-called " rare elements," with which for the last 

 seven or eight years I have been specially occupied. 



Table. 



Didymium 



Decipium 



Samarium 



Lanthanum 



Erbium 



Philippium , 



Holmium 



Thulium 



Dysprosium 



Yttrium 



Terbium 



Gadolinium (Yo) 



Ytterbium 



Scandium 



1707. Tm^Os. 



88 -9, Yt,03. 

 1247, TbPa. 



173-01, Yb.O,,. 

 44-03, ScPg. 



Atomic Weight of Metal and 

 For i.ula of Oxide. 



' Ntodymium— 140'3, Nd^Og 

 Praseodym.ium — 143 '6, '?x^0^ \ 

 Unnamed. 



150-12, Sm^Og. I 



138, LaPa. 

 166, Er^Oj. 

 45-48, PpO. 



Gives Spectrum by 



Absorption. 



Absorption and 

 Phosphorescence. 



Phosphorescence. 



Absorption and 

 Phosphorescence. 



Phosphorescence. 



Absorption. 



Absorption. 

 Absorption. 



Phosphorescence. 



Phosphorescence. 

 Phosphorescence. 



^457(1 



G9 



A 550 Era 



A 493 ErjS 



: Xa 

 )Xy5 



( X5 



/ Tma 

 lTmj3 



(XC 



-448 ' Xf 



{Xr, 



Ga 



Gy8 



G5 

 GC 

 Gt, 



G;8 

 GC 



™?l Lecoq de 

 Yt 



Column I gives the names by which they are com- 

 monly known. Column 2 gives their atomic weights, 

 &c. Column 3 shows in what manner they come under 

 the domain of spectroscopy ; and columns 4 and 5 notify 

 the components or raeta-elements into which some of 

 these bodies have been decomposed in 1886 by myself, 

 and in 1887 by Kriiss and Nilson. In the first column I 

 have exercised a judicial leniency in retaining candidates, 

 for the sake possibly of old associations, when strict 

 Justice would have disestabUshed them. Thus, it may 



be doubted whether decipium, philippium, or gadoliniun^ 

 should have been retained. But since doubts have been 

 cast on the integrity of nearly all the occupants of this 

 column, the line should not be drawn too strictly. 



At first spectroscopic examination was applied directly 

 to substances, natural or artificial, which had not under- 

 gone any special preparation. The idea next occurred 

 of attempting to split up substances supposed to be 

 simple into heterogeneous constituents before appealing 

 to the spectroscope. The refined chemical processes 



