Chemistry and Physics.* 487 



stances, natural or artificial, which had not undergone any special 

 preparation. The idea next occurred of attempting to split up 

 substances supposed to he simple into heterogeneous constituents 

 before appealing to the spectroscope. The refined chemical pro- 

 cesses used for this opei-ation may be summarized under the 

 name of fractionation whether they be fractional precipitations, 

 crystallizations or decompositions. A combination of such deli- 

 cate and prolonged chemical processes with spectroscopic exami- 

 nation applied to bodies showing absorption spectra soon led to 

 important discoveries. In two juxtaposed plates, the author 

 shows the normal didymium absorption spectrum as it was gener- 

 ally recognized down to 1878 compared with the whole of the 

 absorption bands belonging to bodies subsequently separated 

 from didymium by fractionation. In 1878, Delafontaine, by a 

 series of chemical fractionations, separated lrom the didymium of 

 samarskite an earth which he called decipia ; and nine months 

 later Lecoq de Boisbaudran announced the discovery of samarium 

 as a constituent of the samarskite didymium, and showed that 

 samarium is characterized by the bands of decipium together 

 with two additional ones. In 1885, on the other hand, Auer, by 

 fractionally crystallizing the mixed nitrates of ammonium, didy- 

 mium and lanthanum, showed that it was possible in this way to 

 cleave didymium in a certain direction and to separate it into two 

 other bodies, one giving green salts, and called neodymium; and 

 the other giving pink salts and called praseodymium. The spec- 

 trum of the former consists of the whole of the bands in the red 

 with part of the large one in the yellow and the second one in the 

 violet ; that of the latter takes the other part of the yellow band 

 and all the rest of the green and blue. But if these two spectra 

 be subtracted from the old didymium spectrum two bands are 

 left; and hence the lair inference that yet a third body is present 

 in didymium to which these bands are due. In Crookes's own 

 laboratory, moreover, didymium has undergone yet other changes* 

 though it is still doubtful whether neodymium and praseodymium 

 have themselves been decomposed or whether didymium itself is 

 capable of being resolved differently according to the manner in 

 which it is treated. Having worked on the spectrum of didy- 

 mium from allanite, cerite, euxenite, fluocerite, gadolinite, hiel- 

 mite, samarskite, yttrotitanite, etc., the author says : "the further 

 I carry the examination the more the conclusion is forced upon 

 me that didymium must not be regarded as compounded of two 

 elements only but rather as an aggregation of many closely allied 

 bodies." In 1886 he found decided indications of the possibility 

 of depriving didymium of band after band until only the deep 

 line in the blue is left. This single band element he calls Da. 

 Subsequently Kriiss and Nilson by examining rare earths from 

 different sources came to the conclusion that the elements giving 

 absorption spectra and known as didymium, samarium, holmium, 

 thulium, erbium and dysprosium were not homogeneous but that 

 each one contained almost as many separate constituents as it 



