190 W. M. HAMLET. 
rapid surface deterioration, hence the economy of always 
keeping iron tools bright and shining, so that the brighter 
the iron the longer its life. 
Before proceeding to the means of protecting ironwork 
from the ravages of meteoric changes it will be as well to 
consider the position of zinc, bronze, lead and copper. Zinc 
is now extensively used to protect iron from rusting, and 
when a thin film or layer of zinc is caused to adhere to iron 
it is said to be galvanised. Zinc is more electro-positive 
than iron, then why should it not decay sooner than iron? 
Galvanised iron admirably stands the weather, and, provided 
it is not subjected to the action of acid vapours, has a long 
life as a building material. Under normal conditions of 
both town and country life, when a sheet of galvanized 
iron is taken out of its original packing case all clean, 
glistening and bright, it becomes rapidly tarnished ; but 
the tarnish is its own protection, forming a film of compact 
solid varnish that does not crack or peel off, and effectually 
protects the metal from further corrosion. The film of 
tarnish consists of zinc oxide which is insoluble. Under 
the influence of much carbonic acid a basic zinc carbonate 
sometimes forms, which is also insoluble. When a zinc 
building is erected very near the sea shore probably some 
of the halogen compounds of zinc are also present in minute 
quantities, these are actinic, and I think accounts for the 
decided blue appearance of galvanised iron structures built 
close on to the sea shore. 
In the case of galvanized iron we have plain ‘black’ iron, 
then a coating of zinc to protect the iron, and after that 
comes the oxygen and lays on a coat of oxide which pro- 
tects the zinc, a system of proceedings somewhat after the 
manner of ‘this is the house that Jack built.’ 
In the case of bronze, the small amount of hydrogen 
sulphide present in the air is sufficient to produce a black 
