The 'Law of Multiple Proportions' was proposed by Dalton in $1803$. It states that when two elements combine to form more than one compound,the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.
Example $1$: Hydrogen and oxygen form water $(H_2O)$ and hydrogen peroxide $(H_2O_2)$.
In $H_2O$,$2 \ g$ of hydrogen combines with $16 \ g$ of oxygen.
In $H_2O_2$,$2 \ g$ of hydrogen combines with $32 \ g$ of oxygen.
The ratio of the masses of oxygen $(16:32)$ is $1:2$,which is a simple whole number ratio.
Example $2$: Carbon and oxygen form carbon monoxide $(CO)$ and carbon dioxide $(CO_2)$.
In $CO$,$12 \ g$ of carbon combines with $16 \ g$ of oxygen.
In $CO_2$,$12 \ g$ of carbon combines with $32 \ g$ of oxygen.
The ratio of the masses of oxygen $(16:32)$ is $1:2$.
This law supports the existence of atoms because it implies that elements combine in discrete,fixed units (atoms) rather than in arbitrary amounts,leading to fixed ratios of mass.