Manchester Memoirs, Vol. Ixii. (1918), No. 10. 15 



coming together again. The impurities which give rise to this 

 luminescence are frequently quite independent of those which 

 produce the colour effects, since quite colourless crystals sometimes 

 give brilliant thermo-luminescent effects, and deeply coloured 

 crystals may give little or no visible luminescence during discharge 

 of their colour. It is also possible that the approach of the separated 

 particles due to electrical attracting forces may set up light-producing 

 vibrations as successive obstacles in their paths are encountered 

 or passed, while the colour disappears with the final coalescence of 

 these particles. 



It appears to be fairly well established that phosphorescence 

 cannot be produced in a perfectly pure substance. The most 

 brilliant phosphorescent effects are produced in substances such as 

 calcium sulphide by minute quantities of compounds of bismuth, 

 mercury, etc., and this effect is quite destroyed either by largely 

 increasing the impurity or by removing it altogether. 



The authors are of opinion therefore that thermo-luminescence,. 

 cathode-ray colours, exotic colours in minerals, etc., are due to the 

 dissociation of traces of impurity in the bodies concerned and not to 

 decomposition of the body itself. 



The marked differences in the action of /3 and y rays in producing 

 different colours, thermo-luminescence, etc., in certain minerals seems 

 to indicate some essential difference in the nature of these rays 

 other than mere wave-length. 



The radium used in these experiments was the property of 

 the Manchester and District Radium Committee, and to them the 

 authors are indebted for permission to use it. In conclusion, the 

 authors wish to express their thanks to Sir E. Rutherford, Mr. 

 C. J. Woodward, of Birmingham, and Mr. T. H. Hill, of Manchester, 

 for several of the specimens used, and specially to Mr. J. W. Jackson, 

 of the Manchester Museum, for many specimens and for his con- 

 tinued interest and encouragement in this work. 



Most of the specimens referred to have been deposited in 

 the Manchester Museum, and may be seen on application to 

 Mr. J. W. Jackson. 



Electro-Chemical Laboratories, 



Manchester University. 



