CHAP. XIII.] " FRACTIONATION " EVIDENCE. 117 



in the insoluble, and the rest in the soluble, state. The operation 

 must take place slowly, so as to .allow the affinities which, by the 

 nature of the case, are almost equally balanced time to have free 

 play. Let us suppose that two earths are present, almost identical in 

 chemical properties, but differing by an almost imperceptible variation 

 in basicity. Add to the very dilute solution dilute ammonia in such 

 amount that it can only precipitate half the bases present. The dilu- 

 tion must be such that a considerable time elapses before the liquid 

 1 Kjgins to show turbidity, and several hours will have to elapse before 

 the full effect of the ammonia is complete. On filtering we have thfc 

 earths divided into two parts, and we can easily imagine that now 

 there is a slight difference in the basic value of the two portions of the 

 earth, the portion in solution being, by an almost imperceptible amount 

 more basic than that which the ammonia has precipitated. This 

 minute difference is made to accumulate by a systematic process until 

 it becomes perceptible by a chemical or physical test." 



With reference to the result to which this most laborious research 

 had led him, I will quote his own words,* remarking in the first 

 instance that crude yttria from samarskite, gadolinite, cerite, and 

 other similar minerals, is the raw material. The first operation is to 

 free it roughly from earths of the cerium group, which is effected by 

 taking advantage of the fact that the double sulphates of the potasi- 

 sium and the yttrium metals are easily soluble in saturated potassium 

 sulphate solution, while the corresponding double sulphates of the 

 cerium group of metals are difficultly soluble-. 



" No longer than twelve months ago the name yttria conveyed a 

 perfectly definite meaning to all chemists. It meant the oxide of the 

 elementary body, yttrium. I have in my possession specimens of 

 yttria from M. de Marignac (considered by him to be purer than any 

 chemist had hitherto obtained), from M. Cleve (called by him ' purissi- 

 Mium '), from M. de Boisbaudran (a sample of which is described by 

 this eminent chemist as * scarcely soiled by traces of other earths '), 

 and also many specimens prepared by myself at different times and 

 purified up to the highest degree known at the time of preparation. 

 Practically these earths are all the same thing, and up to a year ago 

 every living chemist would have described them as identical, i.e., as 

 the oxide of the element yttrium. They are almost indistinguishable 

 one from the other, both physically and chemically, and they give the 

 phosphorescent spectra in menu with extraordinary brilliancy. This is 

 what I formerly called yttria, and have more recently called old yttria : 

 Now these constituents of old yttrium are not impurities in yttrium 

 any more than praseodymium and neodymium (assuming them really 



* Chemical Neu-s, vol.liv, p. liOO. 



