LUMINESCENCE AND INCANDESCENCE 25 



This is thermoluminescence. Diamond, marble, and fluo- 

 rite are examples. Only certain varieties of fluorite show 

 the phenomenon well. A crystal of one of these varieties 

 heated in the bunsen flame on an iron spoon will give otf 

 a white light long before any trace of redness appears 

 in the iron. Other crystals may luminesce in hot water. 

 In all, this luminescence is dependent on a previous illu- 

 mination or radiation of the crj^stal. If kept in the dark 

 for a long time no trace of light appears when fluorite is 

 placed at a temperature of 100°, but after a short exposure 

 to the light of an incandescent bulb, although no light can 

 be observed in the fluorite at room temperature, (luite a 

 bright glow appears at 100°. Calcium, barium, strontium, 

 magnesium and other sulphates containing traces of man- 

 ganese sulphate, show a similar phenomenon after ex- 

 posure to cathode rays (Wiedemann and Schmidt, 1895 

 b). They emit light during bombardment, but this soon 

 ceases when the rays are cut off. If the sulphates are now 

 heated they give off light, red in the case of MgS04 + 

 MnS04, green in the case of CaS04 + MnS04. The power 

 to emit light on heating may be retained for months after 

 the exposure to cathode rays. The emission of light by 

 bodies after previous illumination or radiation is called 

 phosphorescence and will be considered below. It would 

 seem that the cases of thermoluminescence with which we 

 are acquainted are really cases of phosphorescence inten- 

 sified by rise of temperature. The spectrum of thermo- 

 luminescent bodies, also, is similar to that of phosphores- 

 cent ones. (See Fig. 3.) However, not all phospho- 

 rescent materials are also thermoluminescent. The pro- 

 duction of light by animals is quite another phenomenon 

 from thermoluminescence. 



