96 STUDIES IN LUMINESCENCE. 



If there are two bands, it is natural to expect some indication of their exist- 

 ence in the curves of thermo-luminescence. Referring to Fig. 92, there will 

 be found evidence of two bands. After the intensity has reached a maximum, 

 it falls off rapidly at first, then there is an indication of a slowing up in the 

 decay followed by relatively rapid decay. Whether there is a complete 

 bending back in any of the curves can not be stated, because the points are 

 too far apart. Whether a complete bending back is possible depends on the 

 relative intensities of luminescence of the two bands and the relative rapidity 

 of decay. It has not seemed advisable to search for more certain indications 

 of two bands in the thermo-phosphorescence because of the difficulty of 

 increasing the number of observations taken in a given time. In part II 

 of this chapter, curves will be shown in the case of Balmain's paint, which 

 decays slowly enough to be more completely studied. This substance has 

 two bands in its phosphorescence spectrum, and the curves of thermo- 

 luminescence show two maxima under certain conditions of excitation, 

 temperature, and delay in heating. 



EXPERIMENTS WITH BALMAIN'S PAINT. 



In the experiments with Balmain's paint the apparatus used was the same 

 as that used in the study of Sidot blende. A mercury lamp was used to 

 excite the calcium sulphide, which was in powder form. Since the lamp was 

 made of ordinary glass and the light from it was reflected at a mirror before 

 reaching the powder, the excitation was chiefly due to the visible spectrum 

 in the mercury arc. Before each excitation the powder was exposed to 

 infra-red rays in an attempt to bring it to a standard condition. 



The action of infra-red rays upon the phosphorescence of calcium sulphide 

 is not so strong as upon Sidot blende. In the latter case, as has already been 

 shown in Chapter V, phosphorescence due to a very long excitation can be 

 destroyed almost immediately by infra-red rays, and only the highest allow- 

 able temperature, just under dull red heat, is able to produce thermo- 

 luminescence without renewed excitation. In the case of calcium sulphide 

 a very long exposure to infra-red rays was necessary to destroy the phos- 

 phorescence, and no exposure was found to be long enough to suppress the 

 thermo-luminescence completely. At the beginning of the experiments 

 on calcium sulphide this fact was not recognized. If it had been, the 

 powder could have been brought to an approximately standard condition 

 by heating to a temperature a little higher than the highest temperature 

 at which thermo-luminescence was to be studied. Fortunately, in every 

 case the powder was exposed for one minute to infra-red rays of constant 

 strength, hence it was always brought to a semi-standard condition. No 

 extended attempt was made to compare the two methods, since the blue 

 phosphorescence of calcium sulphide is a difficult color to measure in the 

 photometer. 



The fluorescence spectrum of this sample of calcium sulphide is shown 

 in Fig. 98. This curve was obtained by comparison with the light reflected 

 from the surface of a block of magnesium carbonate illuminated by an 

 acetylene flame. Two bands are indicated by the curve, one with a maxi- 

 mum at about 0.41 /* and the other with a maximum at about 0.54^. 



