312 
PHYSICS: NICHOLS AND HOWES 
Such color changes in other substances have been shown to be due to the 
existence in the phosphorescence spectrum of overlapping bands having dif- 
ferent rates of decay and the striking display would seem to indicate an unusual 
complexity of composition. We were surprised, therefore, to learn from Mr. 
Andrews that for the production of this brilliant white phosphorescence cad- 
mium phosphate of exceptional purity is necessary, and that there is no ad- 
mixture of other metallic salts, as in the preparation of the phosphorescent 
sulphides. 
Curves exhibiting the new vanishing type of decay, above described, resemble 
nothing in the earlier work on phosphorescence so much as the curves ob- 
tained as a result of the action of infra-red light on Sidot Blende. 1 Fig- 
ure 7 exhibits this phenomenon. Processes 1 and 2 together show the pure 
decay curve, without infra-red, but if the infra-red is applied after 18 seconds 
of decay the more rapid process 3-4 ensues. If, however, the infra-red is 
removed at the end of 25 seconds the decay resumes a slower rate, as shown by 
process 5. Processes 1, 3 and 4 taken together show a striking resemblance 
to the vanishing type of decay. Attempts to hasten the decay of calcite or of 
the phosphorescent uranyl salts by infra-red action however have yielded no 
result. Here the successively steeper processes must be due to a change in 
the rate of recombination of the ejected electrons with the active phosphores- 
cing groups. It is commonly believed that only certain groups of particles 
are active, the majority being considered to be inactive. It is evident that 
these changes in rate of decay are inherent with the crystal and must be due 
to a more or less sudden change in the internal conditions. The electric 
fields within a crystal are so strong that it is perhaps not surprising that the 
infra-red field, applied from the outside, cannot affect either the phosphores- 
cent action in the active groups or increase the number of free electrons. The 
sudden change in the rate of decay may be due to a change in the electric 
field concomitant with the changed orientation of the charged particles, which 
drives the ejected electrons in greater proportion to the non-active parts of 
the molecule. 
1 Waggoner, Physic. Rev., Ithaca, (Ser. 1), 27, 1908, (209). 
2 Zeller, Ibid., 31, 1910, (367). 
3 Ives and Luckiesh, Astroph. J., Chicago, 36, 1912, (330). 
4 Trowbridge, C. C, Physic. Rev., 32, 1911, (129). 
6 Kennard, Ibid., (Ser. 2), 4, 1914, (278). 
6 Nichols and Howes, Ibid., 9, 1917, (292). 
7 Wick and McDowell, Ibid., 11, 1918, (421). 
8 To be described in a forthcoming number of the Physical Review. 
9 Nichols and Merritt, Ibid., (Ser. 1), 28, 1909, (349). 
10 Nichols and Merritt, Studies in Luminescence, Carnegie Inst., Washington, Pub.. 152, 
1912. 
