PHOSPHORUS. 93 



The manufacture of phosphorus to-day depends on the con- 

 version of common tricalcium phosphate into acid phosphate of 

 calcium by the action of sulphuric acid: 



Ca 3 2PO 4 + 2H 2 SO 4 = CaH 4 2PO 4 + 2CaSO 4 . 



Tricalcium Sulphuric Acid phosphate Calcium 



phosphate. acid. of calcium. sulphate. 



The soluble acid phosphate of calcium is separated from the 

 insoluble calcium sulphate, mixed with charcoal and sand, and 

 evaporated to dryness; the mixture is then introduced into iron 

 retorts and heated, when the decomposition takes place. By 

 heating the aid phosphate of calcium, water is expelled and 

 metaphosphate of calcium is formed : 



CaH 4 2PO 4 = Ca2P0 3 + 2H 2 O. 



The action of charcoal and sand upon the metaphosphate of 

 calcium at red heat, causes the formation of silicate of calcium 

 and a deoxidation of the liberated phosphoric acid by the 

 carbon : 



2(Ca2P0 3 ) + 2SiO 2 + IOC = 2CaSi0 3 -f 10CO + 4P. 



The phosphorus is liberated in the form of vapor, which is 

 condensed by passing through water, carbonic oxide escaping at 

 the same time. 



Properties of phosphorus. When recently prepared, phos- 

 phorus is a colorless, translucent, solid substance, which has 

 somewhat the appearance and consistency of bleached wax. In 

 the course of time, and especially on exposure to light, it be- 

 comes by degrees less translucent, opaque, white, yellow, and 

 finally yellowish-red. At the freezing-point phosphorus is 

 brittle; as the temperature increases it becomes more and more 

 soft, until it fuses at 44 forming a yellowish fluid, which at 

 290 (in the absence of oxygen) is converted into a colorless 

 vapor. 



The most characteristic features of phosphorus are its great 

 affinity for oxygen, and its luminosity, visible in the dark, from 

 which latter property its name, signifying bearer of light, 

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

 phosphorus has to be kept under water, as it invariably takes 

 tire when exposed to the air, the slow oxidation taking place 



