PHOSPHORUS. 221 



latter property its name, signifying " carrier of light," has been 

 derived. In consequence of its affinity for oxygen, phosphorus has 

 to be kept under water, as it invariably takes fire when exposed to 

 the air, the slow oxidation taking place upon the surface of the 

 phosphorus soon raising it to 50 C. (122 F.) at which temperature 

 it ignites, burning with a bright white flame, and giving off dense, 

 white fumes of phosphoric oxide. The luminosity of phosphorus, 

 due to this slow oxidation, is seen when a piece of it is exposed to 

 the air, and whitish vapors are emitted which are luminous in the 

 dark ; at the same time an odor resembling that of garlic is noticed. 



Phosphorus is insoluble in water, sparingly soluble in alcohol, 

 ether, fatty and essential oils, very soluble in chloroform and in 

 disulphide of carbon, from which solution it separates in the form of 

 crystals. 



Although nitrogen has very weak chemical affinities, while those 

 of phosphorus are extremely strong, yet there is a close resemblance 

 in the chemical properties of these two elements. Both are chiefly 

 either trivalent or quinquivalent; both form compounds corresponding 

 to one another in composition, as also in properties. Thus we know 

 the two gaseous compounds NH 3 and PH 3 ; the oxides N 2 O 3 , N 2 O 5 , 

 and P 2 O 3 , P 2 O 5 . There is also metaphosphoric acid, HPO 3 , corre- 

 sponding to nitric acid, HNO 3 . The chlorides NC1 3 and PC1 3 are 

 known, and many other corresponding features may be pointed out 

 It will be shown later that nitrogen and phosphorus have a great 

 resemblance to the metallic elements arsenic and antimony. 



Phosphorus not only combines directly with oxygen, but also with 

 chlorine, bromine, iodine, sulphur, and with many metals, the latter 

 compounds being known as phosphides. 



Phosphorus is trivalent in some compounds, as in PC1 3 , P 2 O 3 ; 

 quinquivalent in others, as in PC1 5 , P 2 O 6 . 



The molecules of most elements contain two atoms ; phosphorus is 

 an exception to this rule, its molecule containing four atoms. The 

 molecular weight of phosphorus is consequently 4 X 30.77 = 123.08. 



Allotropic modifications. Several allotropic modifications of 

 phosphorus are known, of which the red phosphorus (frequently 

 called amorphous phosphorus) is the most important. This variety is 

 obtained by exposing common phosphorus for some time to a tem- 

 perature of 260 C (500 F.), in an atmosphere of carbon dioxide. 

 The change takes place rapidly when a higher temperature is used 

 and pressure is applied. This modified phosphorus is a red powder, 

 which differs widely from common phosphorus. It is not poisonous. 



