416 



Mr. W. Crookes. 



[June 18, 



absolutely unknown, the difficulties of the problem become enormously 

 enhanced. Insolubility as ordinarily understood is a fiction, and 

 separation by precipitants is nearly impossible. A new chemistry has 

 to be slowly built up, taking for data uncertain and deceptive 

 indications, marred by the interfering power of mass in withdrawing 

 soluble salts from a solution, and by the solubility of nearly all 

 precipitates in water or in ammoniacal salts, when present in traces 

 only. What is here meant by " traces " will be better understood if 

 I give an instance. After six months' work I obtained the earth 

 didymia in a state which most chemists would call absolutely pure, for it 

 contained probably not more than one part of impurity in five hundred 

 thousand parts of didymia. But this one part in half a million 

 profoundly altered the character of didymia from a radiant-matter- 

 spectroscopic point of view, and the persistence of this very minute 

 quantity of interfering impurity entailed another six months' extra 

 labour to eliminate these final " traces " and to ascertain the real 

 reaction of didymia pure and simple. 



Chemistry of the Orange Band- forming Substance. 



At first it was necessary to take stock, as it were, of all the facts 

 regarding the supposed new substance, provisionally termed x, which 

 had turned up during the search for the orange band. In the first 

 place x is almost as Avidely distributed as yttria, frequently occurring 

 with the latter earth. It is almost certainly one of the earthy metals, 

 as it occurs in the insoluble oxalates, in the insoluble double 

 sulphates, and in the precipitate with ammonia. It is not precipitated 

 by sodic thiosulphate, and moreover it must be present in very minute 

 quantities, since the ammonia precipitate is always extremely small, 

 and as a rule x is not found in the filtrate from this precipitate. 



At this stage of the inquiry the chemical reactions of x were much 

 more puzzling than with yttria. At the outset an anomaly presented 

 itself. The orange band was prone to vanish in a puzzling manner. 

 Frequently an accumulation of precipitates tolerably rich in x was 

 worked up for purposes of concentration, when the spectrum reaction 

 suddenly disappeared, showing itself neither in precipitate or filtrate ; 

 whilst on other occasions, when following apparently the same 

 procedure, the orange band became intensified and concentrated with 

 no apparent loss. The behaviour of the sulphate to water was also 

 very contradictory ; on some occasions it appeared to be almost 

 insoluble, whilst occasionally it dissolved in water readily. 



Is "x " a Mixture ? 



A very large series of experiments, which need not here be de- 

 scribed in detail, resulted ultimately in establishing the remarkable 

 fact that the x I sought was an earth which of itself could give no 



