Photographic Researches on Phosphorescent Spectra. 239 



The above method of fractionation is not so effectual if more than 

 two bodies are present. In that case the process fails, in any reason- 

 able time, to yield practically pure specimens of more than two out of 

 a group of closely allied earths. Thus, if there are three earths — say, 

 A, B, and C — whose positions in reference to the chemical process em- 

 ployed are in the written order of sequence, we may get a specimen of 

 A as nearly as we please free from B and C, and a specimen of C as 

 nearly as we please free from A and B, but we cannot get a specimen 

 of B practically free from A and C. The law seems to be that to ob- 

 tain practically pure specimens of three closely allied earths, it is essen- 

 tial to have recourse to at least two different chemical processes. The 

 mere continued repetition of the same process will not do, unless indeed 

 the operations are repeated such a vast number of times as to make the 

 approximate expressions no longer applicable, even though the sub- 

 stances are chemically very close. 



For this and other reasons it is advisable to change the method of 

 fractionation after one process has been in operation for some time. 

 It is evident that any process of fusion, crystallisation, or precipitation 

 can only divide the mass of material into two parts, a soluble and 

 insoluble portion, crystals and mother liquor ; and after a time a 

 balance of affinities seems to be established, and further fractionation 

 appears to do little good. It is better then to change the operation. 



Following the diagrammatic scheme, the portions of earths contain 

 ing most victorium are collected together and fractionated by the crys- 

 tallisation of the oxalates from a solution strongly acidulated with 

 nitric acid in the following manner : — 



To a boiling aeid solution of the nitrate a small quantity of hot 

 solution of oxalic acid is added. The solution remains clear, and it is 

 only after vigorous stirring that a small quantity of insoluble oxalate 

 is formed. The whole is thrown on a hot-water filter and slightly 

 washed with boiling water. To the boiling nitrate a fresh lot of hot 

 solution of oxalic acid is added, and stirred till more insoluble oxalate 

 comes down. This is again filtered off, and the operations of precipi- 

 tating, stirring, filtering, and washing are repeated, always keeping 

 the temperature as near the boiling point as possible, until the whole 

 of the earths are precipitated. Generally the initial earth is divided 

 by this method of fractionation into from six to twelve portions. Each 

 of these oxalates is dried, ignited, dissolved in nitric acid, and the 

 above-described operations repeated. Photo-spectroscopic tests are con- 

 stantly taken during the progress of this fractionation, and portions 

 are mixed together according to the data thus obtained, as shown on 

 the diagram by the lines joining the fractions ; the object being to 

 avoid lateral spreading as much as possible, and, while concentrating 

 the special line-giving earth, to prevent its too great diffusion over a 

 large number of fractions. When the fractionation by the oxalate 



T 2 



