LUMINESCENCE AND INCANDESCENCE 27 



another hole sees it at the moment it is not illuminated 

 and can thus tell if it is phosphorescing. By determining 

 the rate of revolution of the disks it is easy to calculate 

 how long the phosphorescence persists. 



While relatively few solids phosphoresce after ex- 

 posure to light at ordinary temperature a large number 

 of these acquire the property at the temperature of liquid 

 air. Included in the list are such biological products as 

 urea, salicylic add, starch, glue and egg shells. The ,tem- 

 perature also affects the wave-length and hence the color 

 of the light given off. Usually the higher the temperature 

 the shorter the wave-length, but in the case of some 

 bodies (SrS) the wave-lengths become longer at the 

 higher temperature. 



The best known cases of phosphorescence which occur 

 at room temperature and the group to which the word 

 phosphorescence is commonly applied, are those of the 

 alkaline earth sulphides (BaS, CaS, SrS) and ZnS. An 

 Italian, Vicenzo Cascariolo, is said to have discovered the 

 Bologna stone (BaS04) which, by calcination with char- 

 coal, gave an impure phosphorescent BaS or lapis Solaris. 

 Canton's phosphorus (CaS) was later prepared "by heat- 

 ing a mixture of three parts of sifted calcined oyster 

 shells with one part of sulphur to an intense heat for one 

 hour." Hulme spoke of it as the "light magnet of Can- 

 ton," because of its power of attracting and absorbing 

 light. The pure sulphides do not show this property. 

 Only if small amounts of some other metal such as Cu,. 

 Pb, Ag, Zn, Sb, Ni, Bi, or Mn are present, will the sulphide 

 phosphoresce. One part of impurity in a million is often 

 su£6icient. Such mixtures, together with a flux of NagSOi, 

 Li3(P04)2 or some other fusible salt constitute a "phos- 



