PROTECTION OF IRON AND OTHER METAL WORK. 187 
position with the suitable excitant will, under ordinary 
conditions, lead to the corrosion and the final destruction 
of the more electro-negative element. But why, it will be 
asked, does one piece of Muntz metal for example, decay, 
while another will last for many years? The reason is 
based on the condition of the metal, its uniformity of tex- 
ture, the eutectic condition of the alloy, a subject which I 
hope to discuss, in some researches I have on hand, at 
another time. 
Suppose that in the construction of a wharf or jetty, you 
wish to protect a wooden pile from the action of the sea 
and its myriad forms of marine life. You may sheath it 
with iron, but the iron will very soon disappear; sheath it 
with sheet zinc and this will dissolve away and so leave 
the wood exposed and bare. Once more sheath it with 
yellow metal, and it may begin to corrode in a few months, 
or it may possibly last for many years sound and intact. 
This has been the experience of engineers in different parts 
of the world. 
I pass over the interesting question of the adaptability 
of difierent metals suited to difierent purposes. such as cast 
iron and gun metal used to withstand a static load or com- 
pression: wrought iron to resist dynamic stress and so 
forth, and I proceed to the universally known fact of the 
common rusting of iron, a phenomenon known to us from 
boyhood, and not only so, but known to the boyhood of the 
human race, as witness the many allusions to it in litera- 
ture. The rusting of an iron nail, or a knife in a few hours 
is a common occurrence, but the serious magnitude of its 
possible damage is more apparent in the case of great 
structures, thus it has been recorded for example that no 
less than forty tons of iron rust were scraped off the Menai 
Bridge at one time when cleaning down preparatory to 
re-painting. Another instance, Sir Christopher Wren, in 
