Question

...

Answer 1

Here is some information that can help you answer your questions.. Hope it helps

"Atomic theory is the idea that matter is made up of little units called atoms. When the ancient Greek philosopher Democritus came up with the idea in the 5th century BC, is was originally meant to refer to indivisible units. As of 1897, the British scientist J.J. Thomson discovered that atoms are in fact made up of smaller particles. Today, this theory refers to matter being made up of units that are indivisible only some of the time. Exceptions include plasmas, such as fire, other ionic arrangements, such as those found in the body, radioactive materials, and many more." -
http://www.wisegeek.org/what-is-atomic-theory.htm#didyouknowout

Development of Atomic Theory. The atomic theory, which holds that matter is composed of tiny, indivisible particles in constant motion, was proposed in the 5th cent. B.C. by the Greek philosophers Leucippus and Democritus and was adopted by the Roman Lucretius.

"Scientific evidence that convinces people that matter is made up of tiny particles, namely atoms, include microscopic slides that can be viewed, scientific reports that show extensive study and research, and scientific experiments that disprove matter being made up of anything else". ! -
http://www.chacha.com/question/what-kind-of-evidence-would-convince-you-that-all-matter-is-really-made-up-of-collections-of-tiny-particles

Answer 2

If we go back to the 19th century the English chemist Dalton first put forward his Atomic Theory. He came to this conclusion from two laws . The Law of Multiple Proportions and the Law of Constant Composition which you can google for more detail. His theory did 'fit in' with these laws and the majority of scientists at the time held it to be true.
At the turn of the century most of those who were not convinced became convinced after 'Brownian movement' was discovered. It was noticed that when fine pollen was placed on the surface of water in a Petri dish and examined under a microscope that the particles move randomly 'all over the place'. It was concluded that that they were being bombarded by invisible particles - in fact by individual molecules of water made up of hyrdogen and oxygen atoms.