PROTECTION OF IRON AND OTHER METAL WORK. 191 
coating of sulphide of copper that protects the bronze for 
centuries ever afterwards. 
Aluminium is also blackened, as witness the dome on 
one of our Government buildings in Sydney, and which you 
can see in the specimen of blackened aluminium which is 
here exhibited. 
When lead is exposed to the air in the construction of a 
roof for example, it is very quickly covered with a film of 
oxide which is insoluble and has the good effect of protecting 
the metal against most ordinary conditions, but in the air 
of towns the film is changed to one of sulphide which is 
dense black, and this is why the church steeples of big 
towns and the lead roofs of public buildings are invariably 
black. In the country, however, the film may be white or 
yellowish-white, or with varying minute quantities of 
sulphide becoming a dingy grey, but as in the case of zinc, 
the compounds of lead so formed are insoluble in water and 
thus become preservatives. 
Thus it will be seen that iron differs almost entirely from 
all other metals, for instead of forming an insoluble film or 
continuous coating, the rust cakes and peels and dissolves 
and in itself becomes a disintegrator, so that in a series of 
alternate stages the whole mass of metal is eaten through: 
Indeed so thorough has been this chemical change of oxida- 
tion of iron, that I have had cases of the whole of the solid 
iron being removed and the space filled by earthy matter 
and silica, a cast iron water pipe being so attacked that 
the substituted shell of the pipe could be ground to a powder 
by the pressure of the fingers. Under suitable conditions 
the near neighbourhood of electric light and traction mains 
will accelerate the decomposition of iron so profoundly 
as to lead to the utter destruction of underground water 
mains. The mere existence of conductors at high potential 
anywhere near unprotected water and gas mains is a matter 
