The postulates of Dalton's atomic theory were used to explain earlier observations of the behavior of matter. His postulates were... 

(a)  All matter is composed of small, indivisible particles called atoms.

(b)  All atoms of a given element are identical in mass and properties.

(c)  Compounds are formed by a combination of two or more atoms in definite arrangements in the ratio of small whole numbers. 

(d)  Atoms are not created, destroyed or converted into other kinds of atoms during chemical reactions.  They are simply rearranged into new compounds.

These postulates were accepted for most of the 1800's until new experiments indicated atoms were made up of subatomic particles. These new experiments provided evidence that postulates "a" and "b" were not correct.