RELATIONS TO OTHER SCIENCES 249 



made it certain that erbia was a mixture of several earths, and since 

 that epoch, many researches have followed up this subject. Four 

 elements, yttrium, ytterbium, erbium, and Nilson's scandium, seem 

 proved beyond question. The splendid work of Cleve has shown that 

 there are yet other elements belonging to this group, in particular 

 holmium. From the yttria group, too, Marignac has isolated an earth 

 which has been named by Lecoq de Boisbaudran, oxide of gadolin- 

 ium. 



In spite of the continued efforts of the Swedish school, in spite of 

 the researches of so many authorities, Berzelius, Mosander, Cleve, 

 Nilson, Crookes, Marignac, Lecoq de Boisbaudran, Demargay, Brau- 

 ner, Wyrouboff, and Verneuil, this great problem of the rare earths is 

 far from being finished. The separation of these different oxides re- 

 mains one of the most difficult operations of chemistry, and yet when 

 one compares elements so closely akin as these, one feels what inter- 

 est for science would attach to a complete study of them. 



In short, inorganic chemistry has never ceased progressing; and it 

 has taken advantage of all the discoveries achieved in the other 

 sciences. 



The most striking example of this is spectral analysis. . We may 

 recall that Wollaston had in 1802 indicated the discontinuity of the 

 solar spectrum. Later, in 1815, Frauenhofer studied the darkened 

 rays of the solar radiation, and the luminous rays of certain spectra. 

 Though numerous studies of this subject were made by Brewster, 

 Wheatstone, Alter, Angstrom, Masson, and Pliicker, it was not till 

 Kirchhoff 's great discovery, in 1860, that the perfect correspondence 

 between the luminous rays of different spectra and the black por- 

 tions of the solar and stellar radiations became known. 



Spectral analysis was thereupon inaugurated by Kirchhoff and 

 Bunsen, and its value was immediately demonstrated by their dis- 

 covery of the new elements, rubidium and caesium. Inorganic chem- 

 istry appropriated the new method. Sir William Crookes indicated 

 the existence of thallium, which was isolated soon after by Lamy. 

 Reich and Richter discovered indium. Next came the discovery of 

 gallium. Finally, in the hands of many authorities, Bunsen, Thalen, 

 Cleve, Nilson, Crookes, Lecoq de Boisbaudran, Demargay, Becquerel, 

 Benedicks, this method was applied to that difficult problem of the 

 rare earths. 



The simple phenomenon of reversed lines was to extend the field 

 of analytical chemistry to the limits of the furthest visible stars. It 

 was destined to demonstrate that the same matter was distributed 

 throughout the whole universe. In fact, Kirchhoff detected in the 

 atmosphere of the sun the presence of sodium, calcium, and barium; 

 of manganese, iron, chromium, copper, and zinc. Subsequently, 

 Angstrom and Thalen proved the existence in the sun of hydrogen, 



