5^ CHEMISTRY POR AGklCULTURAL StUDENfS 



to boiling (if the metals are pure). Ignite each metal in the 

 Bunsen flame; note the similarity of the magnesia and the 

 oxide of zinc formed. Test the alkalinity of the oxides by 

 placing a little of each on a slip of red litmus paper and then 

 moistening with water. Lastly, boil some water in a wide- 

 necked Florence flask, and plunge into the steam an ignited 

 piece of one of the metals (preferably magnesium ribbon) held 

 by the crucible tongs. Note that the metal burns brilliantly 

 in the steam, the oxide being formed, while the escaping hydro- 

 gen catches fire and burns at the mouth of the flask. 



Procure a piece of metallic iron. Note that, like magnesium 

 and zinc, it is oxidised by damp air, but that, unlike those 

 metals, the rust exfoliates and exposes a fresh metallic surface 

 to further oxidation. (Iron vessels are, for this reason, often 

 coated with tin which does not oxidise in damp air, or " gal'- 

 vanised " with zinc, the film of oxide forming on which does 

 not exfoliate.) Note that the iron has no action on hot water. 

 It has been already observed that iron heated to a low red heat 

 decomposes steam. Note that the iron will not burn in the 

 air, unless it be heated to a white heat by the oxyhydrogen 

 flame, or unless it be in very small particles in the form of iron 

 filings, these being more easily heated to the temperature of 

 ignition, and presenting more surface for oxidation in propor- 

 tion to their size. 



Study the oxidisability of copper and mercury. Note that 

 these do not oxidise in air in the cold. Heat some copper 

 filings in an open crucible, and note that oxide of copper is 



'=<n — T^ ^; ) 



Fig. 26. 



rapidly formed. Heat a little mercury in a tube (Fig. 26) 

 through which a slow current of oxygen gas is passing, and 

 note that the red oxide is formed on the tube just at the point 

 at which condensation of mercury takes place, i.e,^ just below 



