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- 
bme, 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 lumimosity of phosphorus im air or oxy- 
gwen, 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- 
D 
