BY CLEAVAGE, ETC. 35 



When phosphuret of calcium is thrown into wa- 

 ter, a decomposition takes place, and phosphu- 

 retted hydrogen gas is evolved. Each bubble of 

 this gas, as it comes in contact with the atmo- 

 sphere, takes fire spontaneously, and throws off a 

 ring of white smoke. These pretty rings of smoke 

 are luminous in the dark. 



In the year 1851, M. Petrie discovered that the 

 metal potassium is phosphorescent when exposed 

 to the air, like phosphorus. ( e Annuaire de Millon 

 et Reiset/ 1851.) He covered the potassium with 

 bees' -wax, and then cut it in two. Each segment re- 

 mained luminous for about half an hour, the light 

 being about one-tenth the intensity of that pro- 

 duced by a piece of phosphorus of the same size. 



M. Linnemann published another note in 1859 

 (Journ. fur prak. Chem., Ixxv.), upon the phos- 

 phorence of potassium and sodium, showing that 

 both these metals are luminous upon their freshly- 

 cut surfaces. The light emitted by potassium is of 

 a reddish tint, that of sodium greenish, according 

 to this author. At 60 or 70 (centigrade), the 

 light of sodium is quite as intense, if not more so, 

 than that of phosphorus. I have had occasion to 

 examine sodium whilst phosphorescent. Its light 

 is very feeble at the ordinary temperature of the 

 atmosphere, and ceases when the newly-exposed 

 surfaces of the metal are covered with a layer of 

 oxide (soda). The luminosity lasts for a few mi- 



D2 



