310 
PHYSICS: NICHOLS AND HOWES 
Obviously we have in this spectrum two, and perhaps three overlapping 
bands. A very rapid red band, presumably due to calcite, the band of the 
willemite of short duration and a persistent band of small intensity indica- 
tive of the presence of willemite of long duration. 
Not all specimens of willemite have the complicated curve of decay above 
described. Waggoner, 1 (1908) in his studies of phosphorescence of short dura- 
tion, determined the beginnings of the curves of decay of two will emites; one 
of the rapid and one of the persistent variety. He used the Merritt phos- 
phoroscope having a range up to 0.06 seconds. His curves are of the persistent 
type. 
Not all phosphorescence of quick decay is of the vanishing type. — That the type 
of phosphorescence is not altogether a question of duration we have abundant 
i 
CADMIUM PHOSPHATE. 
U-V. EXC. 
DISK. 
0 
40 
20 
•T 
1.00 SECOND. 
r 
FIG. 
6 
evidence. The willemite of quick decay measured by Waggoner is of much 
shorter measurable duration than the Franklin Furnace calcites, yet its curve 
of decay is clearly of the opposite type. The same is true of various phos- 
phorescent compounds prepared by Waggoner, i.e., ZnCl 2 , CdCl, CdS04, — 
each with a trace of MnSCU added, which were heated to redness with a flux 
of Na 2 S04. Although their phosphorescence is of brief duration they gave 
decay curves of the persistent type. 
There is this distinction between the luminescence of such substance and van- 
ishing phosphorescence. The latter becomes completely extinct almost im- 
mediately after it ceases to be of measurable intensity; as indicated by the 
steepness of the second and third processes (fig. 2). In the persistent type 
