BY CLEAVAGE, ETC. 33 



combines with the oxygen of the atmosphere to 

 form phosphorous and phosphoric acids. As a 

 chemical phenomenon it is in every respect similar 

 to the flame produced when potassium or sodium 

 burns in contact with water, or when many bodies 

 having very strong affinities for each other, com- 

 bine, with a production of light. The phospho- 

 rescence of phosphorus, and the combustion (for- 

 mation of phosphorous and phosphoric acids) by 

 which it is accompanied, occur in air or oxygen 

 gas at a given temperature. But if the pressure 

 of these gases be diminished, the phosphorus 

 becomes luminous at a lower temperature; and 

 reciprocally, if the pressure be increased, the tem- 

 perature must be elevated proportionally to make 

 the phosphorus shine. The introduction of some 

 foreign gas, such as hydrogen, nitrogen, or car- 

 bonic acid, into the mixture, has the same effect 

 upon the luminosity of phosphorus in air or oxy- 

 gen, as if the pressure of the latter were dimi- 

 nished a remarkable phenomenon observed by 

 M. Bellani. This is the reason phosphorus shines 

 at a lower temperature in the air than in pure 

 oxygen gas. 



Thenard has made a curious experiment : he 

 shows that nitrogen, hydrogen, or carbonic acid, 

 which have remained for five or six hours in con- 

 tact with phosphorus, and have then been sepa- 

 rated from it, become luminous when a few bub- 



