26 PHOSPHORESCENCE 
allowed to cool, it cracks or splits up as its tem- 
perature decreases, and, at the same time, emits 
a feeble ight. 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 lght. 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 heht by boracic 
acid: when melted before the blowpipe, and allowed 
to cool im the dark, it cracks and gives a sudden 
flash of hght 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 hg¢ht was produced in a similar manner 
to the electric radiation which is sometimes ob- 
served when a card is suddenly torn asunder after 
bemg 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. 
