finely powdered glass is sometimes added in order to increase the
friction. Unless the head of the match is rubbed on the prepared
phosphorus coating, ignition does not occur, and accidental fires are
avoided.

Various kinds of safety matches have been manufactured in the last few
years, but they are somewhat more expensive than the ordinary form,
and hence manufacturers are reluctant to substitute them for the
cheaper matches. Some foreign countries, such as Switzerland, prohibit
the sale of the dangerous type, and it is hoped that the United States
will soon follow the lead of these countries in demanding the sale of
safety matches only.

39. Some Unfamiliar Forms of Burning. While most of us think of
burning as a process in which flames and smoke occur, there are in
reality many modes of burning accompanied by neither flame nor smoke.
Iron, for example, burns when it rusts, because it slowly combines
with the oxygen of the air and is transformed into new substances.
When the air is dry, iron does not unite with oxygen, but when
moisture is present in the air, the iron unites with the oxygen and
turns into iron rust. The burning is slow and unaccompanied by the
fire and smoke so familiar to us, but the process is none the less
burning, or combination with oxygen. Burning which is not accompanied
by any of the appearances of ordinary burning is known as oxidation.

The tendency of iron to rust lessens its efficiency and value, and
many devices have been introduced to prevent rusting. A coating of
paint or varnish is sometimes applied to iron in order to prevent
contact with air. The galvanizing of iron is another attempt to secure
the same result; in this process iron is dipped into molten zinc,