122 



Mr. W. Crookes. 



[Feb. 17, 



Thorium, as oxide or sulphate, refuses to phosphoresce, and the 

 tube rapidly becomes non-conducting. A tube with thoria at one end 

 and a phosphorescent earth such as lime or yttria at the other end, 

 and furnished with a pair of poles near each end, at a particular 

 exhaustion is non-conducting at the thoria end, while it conducts at 

 the yttria end. If the wires of the induction coil are attached to the 

 poles at the thoria end, no current will pass ; rather than pass through 

 the tube, the spark prefers to strike across the spark gauge — a striking 

 distance of 37 mm. — showing an electromotive force of 34,040 volts. 

 Without doing anything to affect the degree of exhaustion, on trans- 

 ferring the wires of the induction coil from the thoria to the yttria 

 ond, the spark passes at once. To balance the spark in air the wires 

 of the gauge must be made to approach till they are only 7 mm. 

 -apart, equivalent to an electromotive force of 6440 volts ; the fact of 

 whether thoria or yttria is under the poles making a difference of 

 .27,600 volts in the conductivity of the tube. The explanation of this 

 action of thoria is not yet quite clear. From the great difference in 

 the phosphorescence of the two earths, it is evident that the passage 

 of the electricity through these tubes is not so much dependent od 

 the degree of exhaustion as upon the phosphorogenic property of the 

 body opposite the poles. This view is supported by the fact that the 

 thoria may be replaced by a metal wire, when the same obstructive 

 action will result. 



Lime does not give phosphorescent properties to thoria, if this 

 earth be pure, but it brings out the lines of yttrium and samarium 

 which are almost always present in small quantities in thoria unless 

 it has been specially purified. 



Tin with 95 per cent, of lime gives the lime phosphorescence only. 



Thulium and erbium together phosphoresce with a green light, 

 giving the erbium spectrum already described before this Society.* 

 There is, in addition, a faint blue line apparently double (see 

 '"Ytterbium"). The addition of lime causes the mixture to phos- 

 phoresce of a pale blue colour. The spectrum now shows a bright 

 blue band, in the same position as the faint double blue band seen in 

 the absence of lime. The blue line of Go, is also seen, and a faint line 

 of GB. The deep red line of Gr], one of the constituents of the 

 ordinary yttria spectrum, is prominent in this spectrum. 



Tungsten and uranium, each mixed with 95 per cent, of lime, only 

 give the lime spectrum. 



Ytterbium. — I have not yet succeeded in preparing this body of 

 trustworthy purity ; but through the kindness of Professor Cleve, 

 JVL de Marignac, and Professor Nilson I have been enabled to 

 experiment with specimens of ytterbia prepared by these chemists. 



* 'Koy. Soc. Proc.,' vol. 40, p. 77, fig. 1 (January 7, 1886). 



