Mysterious Island

It is 1865 and Union prisoners use a military balloon to escape a Southern prison camp near the end of the Civil War. The balloon drifts for days and finally lands on a mysterious volcanic island with very unusual inhabitants. Also landing, in a better aircraft, is Rulu, a visitor from Mercury. She seeks a radioactive material that will enable her to manufacture an explosive hat will destroy the world or, at least, the portion known as Earth.