306 
PHYSICS: NICHOLS AND HOWES 
Ives and Luckiesh 3 on the effect of temperature upon the curves of decay of 
certain phosphorescent sulphides might seem to demand a modification of the 
usual law in certain cases, while the studies of the phosphorescence of gases 
by C. C. Trowbridge 4 and of paraffine at liquid air temperatures by Kennard 5 
indicate that for the range covered by their measurements the law of decay 
cannot be expressed by a summation of l/(a + bt) 2 terms. 
In a recent investigation 6 we have found the exceedingly brief phosphor- 
escence of the uranyl salts, which, although very brilliant lasts only for about 
0.03 seconds, to be of an entirely different type. It decays very slowly at 
first and later very rapidly, whereas in all cases previously studied the opposite 
is true. Measurements just completed on the luminescence of calcite indi- 
cate that this newly determined form of decay curve is not confined to the 
uranyl salts. 
PERSISTENT TYPE. 
— 
/ / 
/ 
/ 
TIME. 
FIG. 1 
VANISHING TYPE. 
TIME. 
FIG. 2 
We propose therefore to recognize two distinct types of phosphorescence 
and to designate them as persistent phosphorescence and vanishing phosphor- 
escence. They are sharply distinguished from one another by the following 
characteristics. 
Types of phosphorescence. — 1: (Characteristic of persistent phosphorescence.) 
This type has a curve of decay made up of a succession of linear process, of 
diminishing slope as we proceed from the origin of time (fig. 1). Three or 
more such processes have been found in all cases which have been studied 
through a sufficient range. 
2: (Characteristic of vanishing phosphorescence.) This type has a curve of 
decay made up of a succession of linear processes, of increasing slope as we 
proceed from the origin of time (fig. 2). 
