Law of Reciprocal Proportions
The principle that the mass ratios in which two elements combine with a third are related by a simple ratio to the ratio in which they combine with each other.
Unit
Dimensionless
In depth
Proposed by Richter, the law was further evidence that each element has a fixed combining capacity, and it underpins the concept of equivalent weight. It follows naturally from atomic theory, since each element combines in whole numbers of atoms whatever its partner.
Examples in the real world
Hydrogen and carbon combine with oxygen in ratios that predict correctly the 1:3 H:C mass ratio in methane.