Manchester Memoirs, Vol. Ixii. (1918), No. 10. 13 



cathode rays, while the other part remained unaltered. This again 

 indicates that the glow is due to impurity present in the crude 

 mineral. On heating the specimen thus treated, a bluish white 

 glow was observed on the previously inactive part of the mass, 

 while the remainder was quite dark. 



Glass. — The action of radium on glass is well known. Soda- 

 glass is coloured violet by the prolonged action of radium, and when 

 this glass is heated a pale violet light is emitted and the colour is 

 discharged. In this respect its behaviour is very similar to that of 

 purple fluor-spar, but the thermo-luminescence is not so bright. 

 Some samples of treated soda-glass require heating to the softening 

 point before all the colour is discharged. 



Jackson considers that the green phosphorescence of X-ray 

 tubes depends upon the presence of traces of manganese. Without 

 this the glow is faint blue. 



Lead glass acquires a fine brown colour under the action of 

 radium, and this colour is also discharged by heat. Boro-silicate 

 glass acquires a purplish brown colour. Other coloured glasses are 

 described by Doelter. 



Bakelite, a hard resinous condensation product of certain 

 aldehydes and amines, is amber-yellow when freshly made, but 

 gradually acquires a wine-red colour under the action of daylight. 

 If the red' colour is not too strong it may be discharged by heating 

 to 100-150 C. for several hours. One sample tested by the authors 

 was restored to its original yellow colour by twelve hours' heating 

 to 130 C. in an air oven, whilst another more deeply coloured 

 specimen was only slightly affected by three days' heating to 150 C. < 

 Radium or cathode rays produce an effect similar to that of day- 

 light, but the colour thus produced is much more easily discharged 

 on heating. 



This colour change of bakelite has proved objectionable when 

 the substance has been used for ornamental purposes (imitation 

 amber) and has led to its abandonment in certain trades. 



Summary and Conclusions. — There appears to be little doubt 

 that the colours and thermo-luminescent properties of many minerals 

 have been largely determined by the presence of radio-active matter 

 either in the water from which they have been deposited or by the 

 subsequent action of radio-active minerals in their immediate 

 neighbourhood. It appears possible, though not proved, that 

 fluorescence is produced by a radiation as in the case of zircons. 

 Different colours may be produced by (3 and y radiation and all 

 three effects may be observed in one and the same crystal (e.g. the 

 fluorescent purplish green fluor where the green colour is restored 

 by y rays, the purple by (3 rays, and the complete colour by placing 

 in a tube of emanation where a, /3, and y rays act together). 



Both /3 and y radiation appear to be active in producing 

 thermo-luminescence. 



In nearly all cases the colours produced are due to the dis- 

 sociation of minute traces of certain impurities. The products of 

 dissociation are removed to a very short distance from each other, 



