Subject: How did early knowledge of atom, molecules, etc. come about?

If one cannot see the atoms and molecules, how did knowledge of them come 
about? Were early scientists just very insightful, or were certain fomulas 
and/or theories used to prove the existance of said particles? P.S: Question 
comes from myself and my mother, who is finishing her degree in elementary 
education.

Re: How did early knowledge of atom, molecules, etc. come about?