A. Because the lilac tinge is produced by partial oxidation; and the process which forms rust has, therefore, already commenced.
Q. How can lilac steel be kept free from rust?
A. By keeping it in a very dry place; for then no additional oxygen will come in contact with it, to increase its amount of rust.
Q. Do any other metals (besides iron) combine rapidly with oxygen?
A. Yes; copper, lead, mercury, and even silver to some extent.
Q. Why does copper tarnish?
A. The tarnish of copper is caused by its oxidation; that is, the oxygen of the air combines with the surface of the copper, and instead of rusting it, covers it with a dark tarnish.
Q. Why does lead lose its brightness, and become dull and of a darker hue, by being exposed to the air?
A. The vapour of the air combines with the lead, and oxidizes its surface; but instead of becoming rusty, the surface becomes dull, and of a darker colour.
Q. Why is it difficult to keep silver bright?