TRANSACTIONS OF SECTION B. 587 



narrow a slit. In appearance tlie bands are more analogous to the absorption 

 bands seen in solutions of didymium than to the lines given by spark spectra. 

 Examined with a high magnifying power all appearance of sharpness generally 

 <iisappears ; the scale measurements must therefore be looked upon as approximate 



Fig. 1. 



only ; the centre of each band may be taken as accurately determined within the 

 unavoidable errors of experiment, but it is impossible to define their edges with 

 much precision. 



As a general rule, the purer the earth the sharper the band, and when impuri- 

 ties are removed to the utmost exteiit, the sharpness is such as to deserve the 

 name of a line. 



To this rule one exception occurs. The body which I have named SS, or 609, 

 is remarkable for the great sharpness of its phosphorescent line, and I have noticed 

 scarcely any variation in its sharpness, however large the bulk of extraneous earth 

 associated with it. This line, however, is sharper and brighter when the current 

 is first turned on than it is after phosphorescing for a minute or so. 



In the Bakerian lecture on yttrium delivered before the Hoyal Society,^ I 

 described the phosphorescent spectrum given by this element, and in the address 

 which I have had the honour of delivering before this Section I gave a drawing 

 of the spectrum of yttrium, together with a sketch of the train of reasoning by 

 which I had been led to the opinion that excessive and systematic fractionation 

 had split up this stable molecular group into its components, distributing its atoms 

 into several groups, with difierent phosphorescent spectra. 



No longer than twelve months ago the name yttria conveyed a perfectly defi- 

 nite 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. Oleve (called 

 by him ' purissimum '), 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 difierent 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 indis- 

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

 phosphorescent spectra in vacuo 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 praseody- 

 mium and neodymium (assuming them really to be elementary) would be impurities 

 in didymium. They constitute a veritable splitting up of the yttrium molecule 

 into its constituents. 



The plan adopted in the fractionation of yttria does not difier in principle from 

 the methods described in my paper ' On the Methods of Chemical Fractionation.' 

 Dilute ammonia is added to a very dilute solution of the earth in only sufficient 

 quantity to precipitate one half. After standing for several hours the precipitate 



• Phil. Trans., Part III. 1883. 



