Why did Rutherford conclude that an atom's nucleus has a possitive charge instead of a negative charge?

When Rutherford did his experiments the generally held
view of matter was that it contained a mixture of positive and negative particles but
that it was solid, with no space between the individual particles.  Physicists also knew
that like charges would repel and opposite charges would attract and that the closer
together the charges were the stronger the interaction between
them.

Rutherford designed an elegant experiment in which he
used alpha particles, given off in the decay of radioactive isotopes, as his bullets.  
The alpha particles were known to have a positive charge. He set up targets consisting
of thin films of various metals, primarily gold.  Around the film, opposite the "gun" he
placed a flourescent screen that would glow if hit by the alpha
particles.

When he ran his experiments he found that most
of the time the alpha particles went right through the metal film, as if there was
nothing being hit by the particles.  Occasionally the particles would deflect (bend) a
little bit, and very rarely the alpha particles would bounce off at a greater angle or
even bounce back toward the gun.  Rutherford described it as if a cannon ball hit a sail
on a ship and bounced back at the cannon that had fired the
ball.

After numerous experiments and observations he
determined that the metal films were not really solid but mostly empty space.  Because
of the behavior of the alpha particles he also concluded that something inside the film
had a strongly positive charge that was deflecting the alpha particles when they came
close. Furthermore, that positive charge was strong enough to actually stop the flight
of the alpha particle and reverse its direction.

Rutherford
contributed greatly to understanding in greater detail the structure of the
atom.