1887.] 



On Radiant Matter Spectroscopy. 



115 



and it was then increased mitil the line next visible was seen. In 

 this way the minimum speed of revolution necessary to bring each 

 line into view was obtained, and from these data the duration of 

 phosphorescence for each constituent of yttria was calculated. The 

 time in the following table represents in decimals of a second the time 

 elapsing between the cessation of the induction dischar/ge and the 

 visibility of the residual glow of the earth : — 



At 0'0035 sec. interval the green and blue lines of Gr/8 and Gfa begin to be visible. 



At 00032 „ the citron line of Gr<5 begins to be visible. 



At 0-00175 „ the deep red line of (K (647) is just visible. 



At 0'00125 „ the line of Gr<5 is almost as bright as that of GryS, and the 



red line of Gr// is visible. 

 At 0"000875 the highest speed the instrument could be revolved with 



accuracy, the whole of the lines usually seen in the 



yttria spectrum could be seen of nearly their usual 



brightness. 



I have already recorded* that phosphate of yttria, when phospho- 

 resced in vacuo, gives the green lines very strongly whilst the citron 

 .band is hazy and faint. The same tube of yttric phosphate was now 

 .examined in the phosphoroscope. The green lines of G/3 soon showed 

 themselves on setting the wheel into rapid rotation, but I was unable 

 ,to detect the citron band of G5 even at a very high speed. 



The effect of calcium on the phosphorescence of yttria and samaria 

 has been frequently referred to in my previous papers. It may save 

 time if I summarise the results here. About 1 per cent, of lime added 

 to a badly phosphorescing body containing yttrium or samarium 

 always causes it to phosphoresce well. It diminishes the sharpness 

 of the citron line of GS but increases in brightness. It also renders 

 the deep blue line of Gra extremely bright. The green lines of G/3 

 are diminished in brightness. Lime also brings out the phospho 

 rescence of samarium, although by itself, or in the presence of a small 

 quantity of yttrium, samarium scarcely phosphoresces at all. 



In the phosphoroscope the action of lime on yttrium is seen to 

 entirely alter the order of visibility of the constituents of yttrium. 

 In a mixture of equal parts yttrium and calcium, the citron GrS line 

 is the first to be seen, then comes the Ga blue line, then the G/3 green 

 line, and finally the G?/ red line. This may, I think, be explained 

 somewhat as follows : — Calcium sulphate has a long residual phos- 

 phorescence, whilst yttrium sulphate has a comparatively short 

 residual phosphorescence. Now with yttrium, although the green 

 phosphorescence of G/2 lasts longest, it does not last nearly so long as 

 that of calcium sulphate. The long residual vibrations of the calcium 

 compound induce, in a mixture of calcium and yttrium, phospho- 

 rescence in those yttric molecules (Gd) whose vibrations it can assist, 

 * ' Phil. Trans.,' 1883, Part III (pp. 914—916). 



