Law of definite composition was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight. Proust worked with two samples of cupric carbonate; one of which was of natural origin and the other was synthetic one. He found that the composition of elements present in it was same for both the samples as shown below:
Thus, irrespective of the source, a given compound always contains same elements in the same proportion. The validity of this law has been confirmed by various experiments. It is sometimes also referred to as Law of definite composition.