102 STUDY IN LUMINESCENCE. 



law after it has reached its maximum intensity, then the discussion pre- 

 viously given still holds good, the supposition being that the second band 

 is completely formed before the first point on a curve can be measured. If, 

 however, conditions can be changed so that a point can be observed before 

 the second band is completely formed, then the curve will show initially 

 an upward bending, as in curve 2, Figs. 99 and 100. If a band is formed 

 by some secondary effect it will usually be present in the fluorescence 

 spectrum because the excitation is continued long enough for the band to 

 be formed. 



The effect of previous history can be explained without adding any com- 

 plications to the ideas already set forth. In the case of Sidot blende, it 

 was undoubtedly the band of longer wave-length that decayed more slowly. 

 The same is true, probably, for calcium sulphide, for the color changes 

 from blue to green when the powder is heated. In the case of the decay 

 curve at room temperature, the intensity is too small for the change in 

 color to be recognized by the eye. Assuming that the band of longer 

 wave-lengths decays more slowly, and that this band is due to some 

 secondary effect, then the action of repeated excitations without inter- 

 vening treatment with infra-red might be to increase the intensity and to 

 decrease the rapidity of decay of the second band without introducing any 

 change in the rapidity of decay of the first band. Some indications of 

 these effects can be seen in Figs. 10 1 and 102. 



It is unfortunate that the curves in Figs. 101 and 102 were not taken 

 in the reverse order, i. c, excitation of long duration followed by shorter 

 excitations. Such curves, according to the ideas just set forth, should 

 show parallelism after the decay had proceeded some time. Several of the 

 sets of curves described in Chapter IV were taken in this order and they 

 exhibit parallelism after decaying a short time. To explain completely 

 the curves in Figs. 99 and 100 on the basis of a second band due to secondary 

 causes is impracticable, because the problem is complicated by the addition 

 of the effect of temperature. One assumption will necessarily have to be 

 added, i. c, that there are certain temperatures at which either one or the 

 other of the bands is most readily formed. This is a possible explanation 

 of the fact that the decay curve becomes a straight line at one temperature. 



A number of sets of observations were made to show the effect of heating 

 after decay has begun. In these experiments the powder was exposed to 

 infra-red rays, then excited and allowed to decay at room temperature, heat- 

 ing being begun at a definite point on the decay curve. Figs. 107 and 108, 

 in which the coordinates are intensity and time, show the effect of varying 

 the length of excitation. The double flash mentioned in the case of Sidot 

 blende is plainly evident in the curves, and in the experiment it was evident 

 to the unaided eye. The color also varied, being blue for the first flash and 

 a yellowish green for the second. At the longest excitation the second 

 flash is barely visible in the photometer, the only evidence of its existence 

 being a slowing up in the rate of decay. As the length of excitation is 

 decreased, the second flash becomes, relative to the first, greater and greater. 



At lower temperatures the second flash becomes less evident, and this 

 peculiarity of the curve for 600 seconds excitation, as shown in Figs. 107 

 and 108, no longer exists. For the lower temperatures the second flash for 



