others tarnish rapidly. This is especially true of marcasite, the ortho-rhombic form of iron sulphide, which is sometimes confounded with pyrite. This even decomposes and crumbles away in time. Pyrite was used to quite an extent in earlier times for mirrors, large pieces of it being ground and polished until they gave a good polish. Among the remains of the Incas of Peru have been found large numbers of these pyrite mirrors.
	190