John Dalton determined that each chemical element is composed of a unique type of atom that atoms differed by their masses.  He devised a system of chemical symbols and, after measuring the relative weights of atoms, arranged them into a table.  His "atomic theory" with spherical solid atoms was based on these measured mass properties.  Dalton also theorized that compounds are composed of molecules.  In turn, molecules are composed of atoms in indefinite proportions.  This led to the laws of definite and multiple proportions.

Links to John Dalton: http://www.slcc.edu/schools/hum_sci/physics/whatis/biography/dalton.html      Biography http://dept.english.upenn.edu/~jlynch/Frank/People/dalton.html      Biography http://webserver.lemoyne.edu/faculty/giunta/dalton.html      Dalton's Atomic Philosophy

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Jessica Gee, Jessica Wong...last updated 2 June 2003