1881.] Discontinuous Phosphorescent Spectra in High Vacua. 209 



and yellow. A phosphorescing" crystal of spodumene has all the 

 internal light cut off with a Nicol prism, when the long axes of the 

 Nicol and the crystal are parallel. 



It became of interest to see if the other earths would show phos- 

 phorescent properties similar to those of alumina, and especially if any 

 of them would give a discontinuous spectrum ; considerable interest 

 attaching to a solid body whose molecules vibrate in a few directions 

 only, giving rise to spectrum lines or bands on a dark background. 



(xlucina prepared with great care is found to phosphoresce with a 

 bright blue colour, but no lines can be detected in the spectrum, only 

 a concentration of light in the blue. 



The rare mineral phenakite (silicate of glucinum), sometimes used 

 as a gem, phosphoresces blue like pure glucina. This mineral shows 

 a residual glow after the current is turned off. 



Thorina has very little, if any, phosphorescence. This earth is, 

 however, remarkable for its very strong attraction for the residual gas 

 in the vacuum tube. On putting thorina in a tube furnished with 

 well-insulated poles, whose ends are about a millimetre apart in the 

 centre, and heating strongly during exhaustion, the earth, on cooling, 

 absorbs the residual gas with such avidity that the tube becomes non- 

 conducting, the spark preferring to pass several inches in air rather 

 than strike across the space of a millimetre separating the two poles. 

 It is probable that this strong attraction for gas is connected with 

 the great density of the earth thorina (sp. gr. = 9*4). 



Zirconia gives a very brilliant phosphorescence, approaching in in- 

 tensity that of sulphide of calcium. The colour is pale bluish-green, 

 becoming whiter as the intensity of the discharge increases : no lines 

 are seen in its spectrum. 



Lanthana precipitated as hydrate and ignited shows no phos- 

 phorescence. After it has been heated for some time before the blow- 

 pipe it phosphoresces of a rich brown. 



Didymia, from the ignition of the hydrate, has scarcely any phos- 

 phorescence ; what little there is appears to give a continuous spec- 

 trum with a broad black band in the yellow-green. On examining 

 the light reflected from this earth when illuminated by day or 

 artificial light, the same black band is seen, and with a narrow slit 

 and sunlight the band is resolved into a series of fine lines, occupying 

 the position of the broadest group of absorption lines in the trans- 

 mission spectrum of didymium salts. 



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



Erbia phosphoresces with a yellowish colour, and gives a continuous 

 spectrum, with the two sharp black bands so characteristic of this 

 earth cutting through the green at X 520 and 523. These lines are 

 easily seen in the light reflected from erbia when illuminated by day- 

 light. It is well known that solid erbia heated in a flame glows with 



