98 CHEMISTR V FOR A GRICUL TURAL STUDENTS 



of iron. To convert this into steel, the phosphorus is oxidised by a current 

 of heated air blown through the molten metal in contact with excess of 

 lime. An unstable basic phosphate of lime,* Ca32P04.CaO, is thus pro- 

 duced which, when finely ground to an impalpable powder, constitutes 

 " basic slag," a substance which can be easily dissolved by weak acid, and 

 is therefore "available" as plant food. As this is a basic substance, it 

 must not, when used as a manure, be mixed with nitrogenous guano or 

 sulphate of ammonia, as ammonia gas would be set free and lost. 

 Similarly "superphosphate of lime" must not be kept mixed with nitrate 

 of soda, as, being an acid phosphate, it sets free nitric acid which, owing 

 to the heating of the mass, is volatilised. 



XXIV. SILICA: SAND AND CLAY 



Potassium and sodium have been found to be the most oxidis- 

 able of all the metals examined. So great is their greediness 

 for oxygen that the oxides of almost all elements are attacked 

 by them, potash or soda being formed and the element set free. 

 This reaction may be employed for preparing the element 

 silicon from its oxide silica. Heat some silver sand (nearly 

 pure silica) in a test-tube with a little sodium. Note that a 

 brown non-metallic substance, the element silicon, is liberated. 

 It has been found by experiment that silica contains 46.6 per 

 cent, of silicon and 53.3 per cent, of oxygen. What is the 

 simplest formula that represents its composition ? 



Mix together 20 grams of silver sand with twice its weight of 

 caustic soda, and heat the mixture strongly on a clean iron dish 

 over a hot fire. When steam ceases to be given off and action 

 appears to be complete, allow the liquid mass to cool, lixiviate 

 with water, boil, and then filter the solution oi silicate of sodium 

 produced. Evaporate a part of the solution till a thick syrupy 

 residue of " soluble glass " is obtained. 



* A basic salt is a compound intermediate between a base and a salt, 

 just as an acid salt is intermediate between an acid and a salt. 



