io8 CHEMISTRY FOR AGRICULTURAL STUDENTS 



piece of glass tubing, and heat. Note the deposit of arsenic 

 formed upon the sides of the tube. 



Owing to this power of carbon as a reducing agent, it is used in 

 metallurgy. Generally speaking, the metallic ores are roasted to convert 

 the sulphide, hydroxide, or carbonate of the metal, as the case may be, into 

 oxide, and then heated with charcoal, coal, or coke, either with or without 

 a fusible compound to act as a flux, until the oxide is reduced to the metal. 

 Iron, copper, antimony, manganese, tin, zinc, sodium, and potassium, are all 

 separated from their oxides by reduction by carbon. In some other cases, 

 e.g. magnesium, which is obtained from its chloride, the metal sodium is 

 used as a reducing agent, while in the case of lead and mercury, which are 

 obtained from their sulphides, no reducing agent is necessary, careful 

 roasting serving to burn off the sulphur as sulphur dioxide. Aluminium is 

 obtained^ by the electrolysis {i.e. decomposition by electricity) of fused 

 cryolite, a native double fluoride of aluminium and sodium. 



XXVII. THE CARBONATES 



Determine the proportion of Hme and carbonic acid gas in 

 chalk. Into a weighed capsule provided with a lid place 

 about 2 grams of pure chalk, dry by warming in the hot 

 water oven for a few minutes, cool in a dessicator, and weigh. 

 Now heat to a full red heat until no further loss in weight 

 occurs, then cover the capsule, cool in a dessicator, and weigh 

 again. From the resulting weight of hme find the percentage 

 composition of chalk, and express this by the simplest formula 

 possible. Calculate what weight of quicklime should be 

 obtained by burning loo tons of chalk in a kiln. What weight 

 of slaked lime would this produce ? 



Carbonates occur abundantly in nature : thus magnesian limestone is 

 carbonate of magnesium ; dolomite is a double carbonate of calcium and 

 magnesium ; malachite is a double carbonate and hydroxide of copper ; 

 calamine is the carbonate of zinc ; clay ironstone is the carbonate of iron. 



