322 SMITH'S INTERMEDIATE CHEMISTRY 



phide, some dextrin and a filling (e.g., chalk). The box is coated 

 with a mixture of red phosphorus, antimony trisulphide, dextrin 

 and filling. The friction converts a trace of the red phosphorus 

 into the white variety, and the latter sets fire to the head. 



Phosphorus Pentoxide j^Os. When phosphorus is burned 

 in dry air or oxygen, under a befi jar, the cloud of pentoxide slowly 

 settles as a white powder. If the pentoxide is thrown into cold 

 water, chemical union takes place. The heat developed pro- 

 duces a hissing sound, caused by the formation and condensation 

 of minute bubbles of steam. The solution, when concentrated at 

 a low temperature, gives crystals of orthophosphoric acid H 3 PO 4 

 (m.-p. 42). 



P 2 O 5 + 3H 2 O - 2H 3 PO 4 . 



On account of its tendency to unite with water, the pentoxide 

 is used for drying gases. 



When phosphorus is burnt in moist air, the cloud of pentoxide 

 forms tiny droplets, consisting of a concentrated solution of phos- 

 phoric acid, which remain suspended in the atmosphere as a fog 

 (compare sulphur trioxide, p. 263) . Burning phosphorus was there- 

 fore used in the war for screening the movement of vessels. In 

 land warfare, shells containing white phosphorus were also em- 

 ployed for incendiary purposes. Such shells produced terrible, and 

 usually fatal, burns on any enemy within the radius of their explo- 

 sion. 



As a tribasic acid, phosphoric acid H 3 PO 4 gives three series of 

 salts by interaction with bases. In the^orthogr normal phosphates, 

 such as Na 3 P0 4 , all of the hydrogen is replaced by a metallic 

 radical. Two series of acid salts are also known (see p. 277). 

 The phosphates of calcium are of particular value as fertilizers 

 (see p. 411). 



Test for a Phosphate. Most phosphates (and phosphoric 

 acid), when mixed intimately with dry sodium carbonate, char- 



