308 
PHYSICS: NICHOLS AND HOWES 
Both types may occur in a single substance. — Shortly after the completion of 
our experiments on the vanishing phosphorescence of the uranyl compounds, 
Misses Wick and McDowell 7 found that some of the same salts, notably 
K.U0 2 (N0 3 )3 and K 2 U0 2 (N03)4 when exposed to the kathode discharge at 
the temperature of liquid air glowed for many seconds after the close of exci- 
tation with decay curves of the persistent type. At + 20° the effect is either 
absent or excessively feeble. Not all the uranyl salts, even among those 
that are reasonably stable in vacuo, respond to the action of the kathode dis- 
charge to a measureable extent. 
The Franklin Furnace calcites, as we discovered in our recent investigation, 8 
have the same remarkable property, i.e., vanishing phosphorescence under 
photo-excitation and persistent phosphorescence under the kathode discharge. 
Figures 3 and 4 exhibit the vastly different behavior of the same calcite after 
the close of these two modes of excitation. 
Both types of phosphorescence are obtainable with calcite at any tempera- 
ture between -180° and +300°. 
Independence of the two types. — Since the kathode discharge modifies the 
surface layers of substances subjected to its action, it might be supposed that 
calcite crystals after kathodo-bombardment would show persistent phosphor- 
escence when photo-excited. To test this supposition we placed a crystal of 
the Franklin Furnace calcite in the bottom of a V shaped vacuum tube having 
a quartz window. 
It could thus be excited either by the kathode discharge or from an iron 
spark. Previous excitation by the kathode rays had no observable effect on 
the photo-phosphorescence which was of the vanishing type and had approxi- 
mately at least the normal color, brightness and duration. The photo-phos- 
phorescence could be superimposed upon the persistent kathodo-lumines- 
cence, as a fleeting effect, at any time during the life of the latter either after 
or before the close of the kathodo-excitation. Apparently the two were en- 
tirely independent of each other. 
Misses Wick and McDowell had previously made a similar observation in 
their study of the kathodo-phosphorescence of the uranyl salts; i.e. that the 
kathode rays do not render them capable of persistent photo-phosphorescence. 
Apparent occurrence of both types with a single source of excitation. — Wil- 
lemite is one of the most brlliant of luminescent substances. The afterglow 
is commonly fleeting, but masses are occasionally found which exhibit phos- 
phorescence of long duration. Such specimens are persistent under photo- 
excitation and kathodo-excitation alike. The phosphorescence of the other 
variety, unlike that of calcite and of the uranyl salts, is of short duration under 
both kinds of excitation. 
The determination of the decay of phosphorescence of a specimen of the 
latter variety, using the disk phosphoroscope, gave curves like that plotted in 
figure 5. Processes 1 and 2 are of type 1 (vanishing) but these are followed 
by a process 3 of lesser slope. 
