Final answer:
The law of multiple proportions states that the ratios of the masses of elements in compounds can be expressed as small whole numbers. This law provides evidence for the existence of atoms.
Step-by-step explanation:
The law of multiple proportions (also known as Dalton's Law) states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element will always be ratios of small whole numbers. This law provides evidence for the existence of atoms because it suggests that chemical compounds consist of fixed ratios of atoms.
For example, if oxygen and carbon form two compounds, the masses of oxygen that combine with a fixed mass of carbon will be in a whole-number ratio.