282 ELEMENTARY GENERAL SCIENCE CHAP. 



The gas is poisonous, so that care has to be taken that it is not 

 inhaled in an appreciable quantity ; the preparation is best per- 

 formed in a draught cupboard or in the open air. It was seen that 

 the gas loses its property of spontaneous inflammability if allowed 

 to stand over water, and the same effect results if the gas be passed 

 through a cooled receiver before collection. This indicates 

 that this property is not due to the gas itself, but to some 

 other product mixed with it, and separated from it, by either of 

 the above methods. This other product is also a compound of 

 phosphorus and hydrogen, but it has a composition represented 

 by the formula P 2 H 4 , and is liquid at ordinary temperatures. It 

 is, as we should expect, spontaneously inflammable, and its 

 presence in other combustible gases, as the gaseous phosphine, 

 hydrogen, &c., causes them, also, to ignite in air, and become 

 themselves spontaneously inflammable. 



In the retort (Expt. 289) used for the preparation of the gas 

 there remains, together with the excess of phosphorus and 

 caustic potash, a compound known as sodium hypophosphUe. 



Manufacture of Phosphorus. Phosphorus is employed for 

 various purposes, but the greatest quantity is used for the pro- 

 duction of matches, the tips of which consist of a little wax 

 with phosphorus and potassium chlorate. The chief source of 

 phosphorus is the residue obtained on burning bones, which is 

 known as bone ash, and consists of phosphate of calcium, Ca 3 (PO 4 ) 2 . 

 This product is converted into phosphoric acid by treating with 

 sulphuric acid, when the following change takes place : 



Calcium -, sulphuric e calcium -, phosphoric 

 phosphate and acid form sulphate and acid, 

 Ca(PO 4 ) 2 + 3H 2 SO 4 3CaSO 4 + 2H 3 PO 4 . 



The calcium sulphate, being insoluble, is easily separated from the 

 acid, which is then concentrated and heated with powdered coke 

 in cast iron retorts connected with pipes dipping under water, 

 by which means phosphorus is obtained. The reactions for this 

 process may be thus represented : 



H 3 PO 4 =HP0 3 

 4HPO 3 + 12C = 12CO+ 2H 2 + 4P. 



The crude phosphorus so obtained is purified by melting under 

 warm water, the melted phosphorus being then cast, while still 

 under water, in the form of round sticks. 



