PHYSICS. 317 



constant frequency intervals. Neighboring bands overlap to such an 

 extent that they can not be visually distinguished, even at the temper- 

 ature of liquid air. 



(9) Under kathodo-excitation this calcite shows brilHant phosphor- 

 escence of long duration (measurable for 300 to 400 seconds). The 

 form of the curve of decay is that characteristic of all substances of per- 

 sistent phosphorescence thus far determined. 



(10) This persistent phosphorescence, unlike the kathodo-phosphor- 

 escenceof the uranyl salts already referred to, occurs not only at — 180°, 

 but at all temperatures within the range of activity of the calcite. 



(11) As in the case of the uranyl salts, there is complete indepen- 

 dence between photo-phosphorescence and kathodo-phosphorescence 

 in the sense that the one mode of excitation in no way modifies the 

 effect of the other, even where the two are applied simultaneously. 



(12) By means of a special form of disk phosphoroscope, which 

 could be operated in vacuo, the earliest stages of the kathodo-phos- 

 phorescence were investigated and it was shown that from the close of 

 excitation to the time at which, by the ordinary methods, observations 

 begin (say half a second), the form of the curve of decay is essentially 

 different from that pertaining to photo-phosphorescence. 



Two Types of Phosphorescence. 



The discovery that the brief photo-phosphorescence of calcite from 

 Frankhn Furnace, like the still briefer photo-phosphorescence of the 

 uranyl salts, follows a law of decay directly opposite to that of other 

 phosphorescent substances thus far investigated, and that the law of 

 decay of the kathodo-phosphorescence of the above calcite and of the 

 uranyl salts is the same as that previously determined for phosphor- 

 escence in general, has led us to define two types of phosphorescence. 

 These we term vanishing phosphorescence and persistent phosphorescence. 



The distinction is not one of duration alone. The modes of decay 

 are altogether different. In persistent phosphorescence the diminution 

 of brightness is very rapid at first and becomes progressively slower 

 in each of the successive processes of which the curve of decay is made 

 up. In vanishing phosporescence the initial rate of decay is relatively 

 slow, but the diminution of brightness becomes progressively more 

 rapid in each successive process. The curves of decay of the two 

 types resemble each other in that they consist of a series of distinct 

 parts or processes, merging into one another more or less abruptly, 

 and that during each process the reciprocal of the square root of the 

 intensity is a linear function of the time. 



The Luminescence of Willbmite. 



The phosphorescence of this brilliantly luminescent mineral is 

 commonly of such brief duration as to elude observation unless the 

 phosphoroscope be employed. Occasional specimens glow for a long 



