Periscope of Medical and General Science in their Relations to Dentistry. [Volume: 2, Issue: 12, July, 1861, pp. 681-694]

692

THE DENTAL COSMOS.

an expedient which must necessarily fail in the end, like every other expedient which violates a law of nature. The history of ships' copper
sheathing shall be taken as our first illustration of the sacrificial metallic
function. Sir Humphrey Davy, as is well known, devised a method of
checking, or altogether obviating the destruction of ships' copper sheathing. Ile accomplished this by attaching to the ship's bottom, at suitable
intervals, slabs of the metal zinc, and which he called protectors. Protectors they were, in the following manner: The zinc rapidly corroded,
and was lost; but, so long as any zinc remained for the sea-water to act
upon, the copper remained untouched. So far as copper preservation is
concerned, the method adopted must be pronounced efficient. Practically,
it failed, indeed, to secure the advantages described, but not for any
reason that concerns us here. It was found that when copper no longer
slowly dissolved, it ceased to be a poisonous metal. Barnacles and seaweed attached themselves, just as they might have done to an uncoppered
wooden bottom; and, moreover, owing to a galvanic action set up, the
ship's copper was rapidly fouled by a deposition of magnesia and lime,
precipitated from the magnesian and calcareous soluble salts always
present in sea-water. Failing, then, to achieve what was intended of it,
the copper-protecting process of Davy ceased to be employed; but the
failure of it is that which alone concerns us here, as illustrating what we
would wish to convey by the words 'sacrificial metal.'
"Take, as the next example, the illustration afforded by the setting of
an iron paling-rail in a bed of lead. The most casual observation, as one
passes along the streets of this city, may be made more pregnant with
fact bearing upon this matter than the longest homily. For a time, varying as to length with locality, external influences, and-perhaps, in some
degree-on the varying quality of the iron and lead brought into contact,
both metals remain sound. But, after a time, decay inevitably sets in,
and, when once commenced, marches to the issue of final destruction with
wonderful rapidity. The remarkable fact is, that both metals do not decay
simultaneously; it is the iron which corrodes, while lead-the softer
metal-remains intact. Let the mechanician do what he will, this result
cannot be prevented-a law of nature having decreed the sacrifice, it must
and will be achieved.
" Our next illustration shall be ordinary tin plate-iron, coated with
tin, as is well known. Well, what sort of destruction is set up when tin
plate has been exposed for a long duration of time to the atmosphere?
Is the tin destroyed-does it tarnish even? Never. Iron is the sacrificial metal here; and, as surely as the iron is anywhere exposed, it perishes, crumbles, and dissolves away with destructive rapidity. The rate
of destruction of iron totally unprotected is slow, by comparison with
that which ensues when, owing to a flaw or disintegration of the covering
tin envelope, atmospheric agencies come into play upon the underlying
metal.
"But the case we most particularly wish to direct attention to, is the
result of bringing zinc into contact with iron, and retaining the two
metals together over long periods, as may be seen in the so-called galvanized iron. Under this latter disposition, zinc becomes the sacrificial
metal. Not one particle of iron decays, so long as atmospheric destructive agents can wreak their dissolution-so to speak-on the protective
zinc. Nor does this protection altogether depend on a complete covering
of the iron. Flaws and imperfections there are, and necessarily must be,
through which, quite down to the iron, destructive agents, always present