Brass and bronze are both alloys of copper -- that is, they are ``solid solutions," or mixtures of copper with another metal.
You're probably used to the idea of liquid solutions, such as salt in water where each atom of salt is surrounded by atoms of water without the two chemically reacting. Here you have much the same sort of thing but with solids closely intermingled.
The main difference between brasses and bronzes is that brass is mostly made of copper and zinc while bronze is mostly made of copper and tin.
Copper is an unusual metal in that, like silver, gold, and tin, it can appear in ``native" form -- that is, as a fairly pure metal, just lying around. Unfortunately, for many applications it's too soft and needs to be combined with other metals to be really useful. This brings us to bronze and brass.
The discovery of bronze dates to about 3500 BC and the Sumerians. It is harder than pure iron and resists corrosion, so it was a good candidate for weapons, with countless people meeting their ends by being struck with lumps of this alloy in one form or another!
Brass took a while longer to discover and dates to about 500 BC. Zinc is practically never found naturally in its pure state, but people had realized that copper smelted with calamine -- a zinc ore -- produced a golden-colored tarnish-resistant metal that was useful for all sorts of things due in part to its low melting point and malleability. The zinc itself is not seen but is released from the calamine ore by heating and combines immediately with copper.
Modern bronzes and brasses can have significant mixtures of other metals to alter their properties, and it's not necessarily easy to tell what's what on sight.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to firstname.lastname@example.org or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown.