42 NATURAL HISTORY SOCIETY OF DUBLIN. 



one of phosphorescence, only because that it in no way implies — at least 

 in respect to the nature of the light emitted by animals — any theory as 

 to the cause of the phenomenon ; but in other respects there can be no 

 objection to the use of the term. Phosphorescence is one of the various 

 sources of light, and has been referred by Becquerel* to five causes : — 



1 . Spontaneous phosphorescence in certain vegetables and animals. 



2. Phosphorescence by elevation of temperature, which is best seen in 

 certain species of diamonds and in fluorspar, which, when heated to 

 300° or 400°, suddenly become luminous, emitting a bluish light. 



3. Phosphorescence by mechanical effects, such as friction, percussion, 

 cleavage, &c. ; for example, when the crystals of quartz are rubbed 

 against each other in darkness, or when a lump of sugar is broken. 



4. Phosphorescence by electricity, like that which results from the 

 friction of mercury against the glass on the tube of a barometer, and 

 especially from the electric sparks proceeding either from an ordinary 

 electric machine, or from a Ruhmkorff 's coil. 5. Phosphorescence from 

 insolation. A large number of substances, after having been exposed 

 to the action of solar light, or of the diffused light of the atmosphere, 

 emit in darkness a phosphorescence, the colour and intensity of which 

 depend upon the nature and physical condition of these substances. The 

 sulphides of calcium and strontium are those which present it in the 

 highest degree. "When well prepared, after being exposed to the light, 

 they are luminous in darkness for several hours, even for as many as 

 thirty hours, whether in vacuo or in a gaseous medium. Besides 

 minerals, a large number of organic substances also become phophores- 

 cent by insolation; for instance, dry paper, silk, cane-sugar, milk-sugar, 

 amber, the teeth, &c. 



"Without dwelling longer on luminosity in the mineral kingdom, 

 we may allude to the occurrence of the phenomenon in vegetables. 

 Certain Fungi, particularly species of Agaricus, have been observed to 

 emit light. Agaricus olearius, not uncommon in Italy, is sometimes 

 seen at night feebly shining amidst the darkness of the olive groves. 

 The mycelium or spawn of the truffle emits light. A Brazilian 

 Agaricus is phosphorescent to such a degree, that two or three of 

 such fungi in a dark room give out a pale-greenish light sufficient 

 to read by ; and Mr. Drummond has described an Australian fungus 

 with similar properties.! Light is perceived in all parts of lumi- 

 nous fungi, but chiefly in the young white shoots ; and it is more 

 vivid in young than in old plants. It ceases if the plant be placed in 

 vacuo, or in any atmosphere which does not contain oxygen, but re- 

 appears when it is restored to the air, even after remaining for some 

 hours in vacuo, or in nitrogen. The Rhizomorpha subterranea, a fun- 

 goid organism occasionally met with in mines, exhibits upon its rami- 



* Ganot's "Physics," 1867, p. 480 



f Carpenter, " Principles of Comp. Phys.," 4th ed., p. 442. Badham, "Esculent 

 Funguses of England," p. 15, 1847. 



