26 PHOSPHORESCENCE 



allowed to cool, it cracks or splits up as its tem- 

 perature decreases, and, at the same time, emits 

 a feeble light. When vanadic acid is melted, it 

 crystallizes on cooling, and during the whole time 

 that this crystallization lasts, the substance glows 

 with a red phosphoric light. When phosphate 

 of lead is melted before the blowpipe, it forms a 

 crystalline bead on cooling, and whilst this crys- 

 tallization takes place light is produced. I have 

 frequently observed the emission of light by boracic 

 acid: when melted before the blowpipe, and allowed 

 to cool in the dark, it cracks and gives a sudden 

 flash of light when it has cooled for about twenty 

 seconds. The acid should be melted upon a pla- 

 tinum wire bent at the end. Berzelius thought 

 that this light was produced in a similar manner 

 to the electric radiation which is sometimes ob- 

 served when a card is suddenly torn asunder after 

 being split at one of its corners. Light is also 

 produced when crystals of sugar and nitrate of 

 uranium* are broken. And I have observed the 

 same to take place with lactine, or sugar of milk. 

 Another kind of sugar, called mannite, presents 

 similar phenomena. An emission of light is also 

 observed when crystals of protochloride of mer- 

 cury (sublimated calomel) are broken between the 

 fingers. 



* The effect is very striking if crystals of nitrate of uranium be 

 shaken up in a bottle. 



