Law of Multiple Proportions (Dalton):

• Definition: If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers.
• Example: In $CO$ ($12\text{ g }C$ with $16\text{ g }O$) and $C{O}_2$ ($12\text{ g }C$ with $32\text{ g }O$), the ratio of masses of oxygen combining with $12\text{ g }C$ is $16:32$, which simplifies to 1:2 (simple whole numbers).