202 STUDIES IN LUMINESCENCE. 



7. Effect of Red and Infra-red Rays. In the case of certain substances 

 the semi-permanent condition produced by excitation may be 

 destroyed and the material restored to a standard state by a brief 

 exposure to the red and infra-red rays. The effect of the longer 

 waves during phosphorescence is to accelerate the decay. In 

 some cases the first effect is to increase the brightness of phos- 

 phorescence, this temporary effect being followed by decay more 

 rapid than the normal. 



THE DECAY OF PHOSPHORESCENCE UNDER SIMPLE CONDITIONS. 



According to the form of the Wiedemann and Schmidt theory that is 

 here adopted, the effect of the exciting light is to produce electric dis- 

 sociation of the active substance and the resulting negative and positive 

 nuclei exist for a time uninfluenced by their mutual attraction. The 

 vibrations that occur upon the recombination of the ions give rise to phos- 

 phorescence. 



The simplest hypothesis regarding the law of recombination of the ions 

 in a luminescent substance is that which has been applied to the case of 

 ionization in gases. 1 Let the number of positive ions present per cubic 

 centimeter at any time t be n. The number of collisions between a positive 

 and a negative ion will be proportional both to the number of positive ions 

 and to the number of negative ions; and a certain fraction of these col- 

 lisions will result in recombination. Since positive and negative ions are 

 present in equal numbers we have 



dn/dt = a n- 1 /n = c-\-at where c = 1 /n 



vSince the intensity of phosphorescence is proportional to the number of 

 recombinations per second 



/ = kan- = ka/(c-\-at) 2 



This is one form of the empirical expression originally proposed by 

 E. Becquerel to express the decay of long-time phosphorescence, and is 

 the same law that was derived on the basis of entirely different theoretical 

 assumptions by H. Becquerel. 2 



In comparing our experimental results with the law just derived, it is 

 convenient to write the above expression in a different form, namely, 



where 



V/ 



a= U b = . ' 



n<Ska ^ 



k 



The decay of phosphorescence in gases appears to be strictly in accord- 

 ance with this law. 3 Under certain special conditions as regards temper- 

 ature, etc., the law is very closely obeyed by zinc sulphide and even by 

 Balmain's paint. 4 But in the great majority of instances the phosphores- 



^utherford. Philosophical Magazine, vol. 44, p. 422, 1897. 



2 H. Becquerel, Comptes Rendus, vol. 113, p. 618, 1891. 



3 C C. Trowbridge. Physical Review, vol. 26, p 515; vol. 32, p. 129. 



4 C A. Pierce, Physical Review, vol. 26, p. 312 and p. 454, 1908. See also Chapter VI. 



