112 Mr. H. M. Vernon on the 



perceptible differences of atomic weight were found between 

 even the extreme members of the fractions. 



We therefore come to the conclusion that the entire grounds 

 upon which the fractionation of yttria into several new earths 

 depends is that of the difference of their phosphorescent 

 spectra. 



Now it may be asked, Is this radiant-matter test to be 

 absolutely relied upon in every respect with regard to its 

 power of distinguishing between different forms of matter ? 

 Does it always give totally different spectra for each form 

 of matter or element as known to us, and does a mixture or 

 compound of two or more elements give the phosphorescent 

 spectrum of each of these elements, or a spectrum different 

 from either? 



It appears that Crookes was the first to use, or at any rate 

 to bring into at all general use, this method of obtaining the 

 phosphorescent spectra of bodies when subjected to an induc- 

 tion discharge in an exceedingly rarefied atmosphere. To 

 him therefore we must look for our knowledge of the relia- 

 bility of this method of examination. He states that the 

 more he makes use of it the more reliable it appears. Do, 

 however, his experiments justify this assumption ? The only 

 bodies which he appears to have examined at all completely 

 by this method are these rare earths, and a few other earths 

 of a similar nature, as alumina. Do, then, the results obtained 

 with these bodies justify this method being used to verify 

 such a gigantic innovation as that of the fractionation of the 

 oxide of what was supposed to be a single element into oxides 

 of several other totally new elements ? Would the following 

 experiment made by Crookes himself seem to justify it ? 



In Phil. Trans. 1885, p. 721, Crookes finds that when 

 varying proportions of samaria and yttria are mixed and 

 ignited with sulphuric acid, and tested by the radiant-matter 

 spectroscopic method, a bright orange-line, 2693, is observed, 

 which does not occur in either of their separate spectra, and 

 which disappears as the proportions in which the earths have 

 been mixed approach either to pure samaria or to pure yttria. 

 When, however, yttria and samaria are mechanically mixed 

 and are not treated with sulphuric acid and ignited, only their 

 superposed spectra, without the orange-line, are obtained. 



Is not the fact of the bright orange-line being present 

 when the earths are ignited with sulphuric acid, and not being 

 present when they are merely mixed, due to the formation of 

 a compound between the two earths in the first case ? It is 

 well known that the spectra of salts which are not decom- 

 posed into their constituent elements by the action of heat are 



