30 LIGHT. 



example.* It gives a vivid emerald green light which continues for 

 a long time. 



Some varieties of marble, heated to a degree that would only 

 make other bodies red, emit an intensely brilliant white light. Tur. 



The dried yolk of an egg becomes luminous if heated, and so 

 does tallow, when thrown on a hot shovel or burning coals ; both shov- 

 el and coals should be rather below redness. Some bodies ceasing 

 to emit light by heat, become again luminous by increase of heat. 



(d.) Some emit light by percussion, friction or pressure. The 

 Dolomite of Litchfield county in Connecticut faintly flashes, when 

 pounded in a mortar ; light is seen when lumps of sugar, or of 

 quartz,f or borax, or bonnet cane, are smartly rubbed or struck to- 

 gether, in the dark, certain varieties of tremolite and of blende give 

 Jight when the point of a knife is drawn across them.J 



(e.) Phosphorescence is seen in some animals. 



The glow worm, and several species of fire fly are examples. The 

 luminousness of the waves of the sea in a storm, or under a vessel's 

 bow, or of water taken from the sea and agitated, is very remarka- 

 ble ; this phosphorescence is owing to animal matter dissolved in die 

 sea water, or to living animals, as the medusa, cancer fulgens,^ &c. 



When the sea water is filtered so as to remove the animals, it is 

 said to lose its phosphorescent power. 



Lit. H. Ingalls, of the U. S. Army, is of opinion that the phospho- 

 rescence of the ocean is owing to the ovula of fishes. He struck 

 his arm, while bathing, against a soft mass, which emitted flashes two 

 qr three inches long, and he even convinced himself that there was 

 a mild degree of heat, grateful to his touch. The jelly like masses, 

 seen upon a beach after the retiring of the tide, he conceives to be 

 the bodies in question ; that these masses are phosphorescent, was 

 proved by their emitting bright light, when irritated by the point of 

 a pencil, especially in a particular opake point, appearing to be the 

 punctum saliens of a living animal which the sun hatches, by de- 

 grees, from the jelly like mass, and the tide eventually shakes out. 

 There is therefore the fullest reason to believe that the luminousness 

 of the ocean is owing to animal matter. || 



Fresh water is not phosphorescent ; the waves of the great North 

 American lakes, although violently agitated by tempests, exhibit no 

 luminous appearance. Air or its absence has no effect on phospho- 

 rescence. 



(/.) Phosphorescence is produced by chemical action. 



* Am. Jour. II, 142. 



t Quartz phosphoresces even under water. Ure. 



t Dr. Brewster's Edin. Phil. Jour. Vol. I. Nicholson's Jour. 8vo. Vols. XV, XVI 

 and XIX. 



Tilloch's Phil. Mag. V. 37. and 38. || Trans, of Albany Institute. 



