922 Sir James Deivar [Jan. 21, 



a vertical beam of violet light reflected in from an arc lamp. A 

 few minute fragments of solid eosin thrown in, gradually dissolve, 

 producing a red solution. The specks of eosin as they fall through 

 the water leave behind a brilliant yellowish fluorescent track which 

 soon permeates the whole solution. The violet rays are absorbed by 

 the eosin, and subsequently emitted in the form of this beautiful 

 greenish yellow colour. 



A somewhat similar fluorescent or phosphorescent effect is pro- 

 duced by some bodies at low temperatures. On the outside of this 

 long test tube is spread some ordinary tgg albumen. This can be 

 cooled down and frozen by simply pouring a little liquid air into the 

 tube. When exposed to the light of the arc the frozen albumen 

 shows a beautiful blue phosphorescence. A second tube covered with 

 glycerin and similarly frozen shows a like effect. Zymase or yeast 

 juice, obtained by breaking up yeast-cells and extracting the liquid 

 contents, shows marked phosphorescence. All culture media used in 

 bacteriological research behave in the same way. An ivory paper- 

 knife cooled by dipping into liquid air phosphoresces brightly. A 

 strip of gelatin behaves in the same manner, so also an ordinary 

 paraffin candle. 



These substances all show phosphorescence at the low temperature. 

 Calcium sulphide on the other hand is a body which will phos- 

 phoresce at ordinary temperatures, and it becomes interesting to see 

 what will be the effect of cooling on this luminous paint. This 

 star-shaped piece of cardboard, which has been coated with calcium 

 sulphide, shows strong pliosphorescence at the ordinary temperature. 

 Now float it on some liquid air in a dish, and you observe the light 

 becomes dim and disappears. Warm it up l)y merely waving it in 

 the air, and very soon it again gives out its characteristic light. 

 This vacuum vessel contains a similar star which has been kept in 

 liquid air for 2-i hours. On taking it out the dark star becomes brightly 

 luminous. Its phosphorescing properties are stored up or rendered 

 latent so long as the low temperature is maintained. In order to 

 becomfe phosphorescent the body must first be exposed to light. It 

 is interesting to see whether cooling to the temperature of liquid air 

 before and during such an exposure has the effect of preventing 

 subsequent phosphorescence. A similar sulphide of calcium card- 

 board star which has not been exposed to light and is consequently 

 non-luminous at the ordinary temperature, is floated in an aluminium 

 dish on the surface of liquid air, and when thoroughly cooled to 

 -185°, exposed to violet light. All remains dark, but on allowing 

 it to warm up we see that light-energy must have been absorbed at 

 the low temperature, because the star now phosphoresces in a marked 

 degree. If this experiment is repeated, using liquid hydrogen instead 

 of liquid air, the same effects are observed. 



All these substances absorb ultra-violet light and transform it 

 into visible phosphorescence. By a similar absorption other bodies, 

 however, possess the power of yielding other forms of energy. The 



