Q. _Why do STOVES and FIRE-IRONS become RUSTY, in rooms which are not OCCUPIED?_

A. Because the air is damp; and moist air _oxidizes_ (or rusts) iron and steel.

Q. _In what part of the year is it most difficult to keep STOVES and FIRE-IRONS BRIGHT?_

A. In _autumn and winter_; because the capacity of the air for holding water _being on the decrease_, its vapour is deposited on every-thing with which it comes in contact.

Q. _Why does GREASING iron prevent its becoming RUSTY?_

A. Because _grease_ prevents the humidity of air from coming in contact with the _surface of the iron_.

Q. _Why do not STOVES rust so frequently as POKERS and TONGS?_

A. Because stoves are generally _covered with plumbago_, or black lead.

Q. _What is plumbago, or black lead?_

A. A mixture of charcoal and iron filings.

A most excellent varnish to prevent rust is made of 1 pint of fat oil varnish, mixed with 5 pints of highly rectified spirits of turpentine, rubbed on the iron or steel with a piece of sponge. This varnish may be applied to bright stoves and even mathematical instruments, without injuring their delicate polish.

Q. _Why does ornamental STEEL (of a purple or LILAC colour) rust more readily than polished WHITE steel?_

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?_

A. Because the vapour of the air oxidizes its surface, and _tarnishes_ it.

Q. _Why do silver TEA-POTS and SPOONS tarnish more quickly than silver ore or bullion?_

A. Because alloy (of some _baser_ metal) is used to make it more _hard and lasting_; and this _alloy_ oxidizes more quickly than silver itself.

Q. _Why does GERMAN silver turn a dingy yellow in a few hours?_

A. German silver has a great affinity for oxygen, and shows its oxidation by a _sickly yellow tarnish_, instead of rust.

Q. _If quicksilver (or mercury) is tarnished like copper and lead,--Why does it preserve its BRILLIANCY in BAROMETERS and THERMOMETERS?_

A. Because _air_ is excluded from it, and no moisture comes in contact with it to _oxidize_ (or _tarnish_ it).

Q. _Is GOLD affected by the atmosphere?_

A. Not readily: gold will never combine with oxygen of itself, (or without aid).

Q. _Which of the METALS is capable of resisting oxidation altogether?_

A. Plat'inum; in consequence of which, the graduated arcs of delicate instruments for observation are made of plat'inum instead of any _other_ metal.

Q. _Why is PLAT'INUM used for the graduated arcs of delicate mathematical instruments, instead of any other metal?_

A. Because it will never oxidize; but retain its _bright surface_ in all weathers free from both _rust and tarnish_.

Q. _Before plat'inum was discovered, which of the metals was employed for the same purpose?_

A. Gold.

Platinum, (a white metal), so called from "plata," the Spanish word for _silver_. It was first introduced into England by Mr. Wood, (A. D. 1749) from South America.

Q. _For what other SCIENTIFIC purposes is PLAT'INUM now used?_

A. For crucibles in which _acids_ are employed, and for galvanic batteries.