268 



Mr. W. Crookes. 



[May 31, 



yttria, proved for a long time the only source of material wherewith 

 to investigate the chemical properties of the body giving the citron 

 spectrum. 



Another reason which made me at this stage of the research pass 

 over yttria was that I had already tested this earth in the radiant 

 matter tube. In a paper on " Discontinuous Phosphorescent Spectra 

 in High Vacua," read before the Royal Society, May 19, 1881, I said, 

 " Yttria shows a dull greenish light, giving a continuous spectrum." 



For these reasons I for a long time omitted yttria from my list of 

 possible bodies, and considered that, the earth, if not a new one, might 

 turn out to be either mosandra, terbia, or- ytterbia. 



About 15 lbs. weight of samarskite- was worked up, partly by the 

 hydrofluoric acid method of Lawrence Smith, and partly by fusion 

 with potassic bisulphate. 



These methods both gave as a result* a large quantity of mixed 

 earths containing 1 most, if not all, the bodies enumerated in the fore- 

 going list. Tested in the radiant matter tube this mixture gave the 

 citron spectrum very brilliantly. 



These earths were treated by a series of chemical processes too long 

 and complicated to describe in this abstract, and the result of about 

 five hundred fractional precipitations gave me a mixture of earths 

 having an H equivalent, M=48, and showing a strong absorption 

 spectrum; a mixture having an H equivalent, M=33, having no 

 absorption spectrum ; and intermediate earths. 



In the radiant matter tube all these fractions gave the citron band 

 spectrum well, but that of the earth of lowest equivalent was much 

 the brightest, and that of the highest equivalent the least intense. 



Three methods are available for the partial separation of these 

 earths and for the complete purification of any one of them. The 

 formic acid process is best for separating terbia, as terbic formate is 

 difficultly soluble in water, the other formates being easily soluble. 



Fractional precipitation with oxalic acid separates first erbia, 

 holmia, and thulia, then terbia, and lastly yttria. This is the only 

 method which is applicable for" the separation of small quantities of 

 terbia from yttria. 



Fusing the nitrates separates ytterbia, erbia, holmia, and thulia 

 from yttria. It is not so applicable when terbia is present, and is 

 most useful in purifying the gadolinite earths. This process is the 

 only one known for separating ytterbia from yttria. 



Selection must be made of these methods according to the mixture 

 of earths under treatment, changing the method as one earth or the 

 other becomes concentrated on one side or thrown out on the other. 

 Each operation must be repeated many times before even approximate 

 purity is attained. The operations are more analogous to the separa- 

 tion of members of homologous series of hydrocarbons by fractional 



