418 



Mr. W. Crookes. 



[June 18, 



orange band spectrum when mixed with lime and treated as usual, 

 although without lime it gave no spectrum. Ultimately, however, a 

 lanthana was obtained which, mixed with lime and treated in the 

 usual manner, gave no orange band spectrum whatever. This lan- 

 thana was snow-white, and had an atomic weight of 138*3. 



Didymia. 



The earth formerly called didymia is now known to be a mixture of 

 didymia and samaria, and systematic operations were now commenced 

 with the object of obtaining the didymia and the samaria in a state of 

 purity — that is to say, in such a condition that one of them should 

 show no orange band spectrum at all, whilst the other should give 

 the spectrum in its highest degree of intensity. 



I commenced the purification of didymia in the latter part of the 

 year 1883, and the operations have been going on since almost daily 

 in my laboratory. At intervals of some weeks the didymia in the 

 then stage of purification was tested in the radiant matter tube, a little 

 lime having previously been added to bring out the discontinuous 

 phosphorescence. During the first month or two the intensity of the 

 orange band spectrum scarcely diminished. After this it began to fade, 

 but the last traces of orange band were very stubborn, and not till the 

 last few weeks could I obtain a didymia to show no trace of the orange 

 band spectrum ; and this result has not been accomplished without 

 sacrifice. My 1000 grammes have dwindled away bit by bit, till now 

 less than half a gramme represents all my store. 



Samaria. 



The foregoing experiments left little doubt that x, the orange-band- 

 forming body, was samarium ; the last problem was, therefore, to get 

 this earth in a pure state. The general plan of operations was the 

 same as I adopted in getting didymium free from samarium, only 

 attention was now directed to the portions richest in samarium which 

 had been formerly set aside. The colour of samaria, as pure as I 

 have been able to prepare it, is white with the faintest possible tinge 

 of yellow. The absorption spectrum of samarium salts is much more 

 feeble than the spectrum of didymium. 



The Phosphorescent Spectrum of Samarium. 



Pure samaric sulphate by itself gives a very feeble phosphorescent 

 spectrum. When, however, the samaria is mixed with lime before 

 examination in the radiant matter tube, the spectrum is, if anything, 

 more beautiful than that of yttrium. The bands are not so numerous, 

 but the contrasts are sharper. Examined with a somewhat broad slit, 

 and disregarding the fainter bands, which require care to bring them 



