12g carbon combines with 64 g sulphur to form CS2. 12 g carbon also combines with 32 g oxygen to form CO2. 10 g sulphur combines with 10 g oxygen to form SO2. These data illustrate the:

Ratio of the weights of S and O combining with fixed weight of C is 64 : 32 = 2 : 1. Ratio of the weights of S and O combining directly = 10 : 10 = I : 1. The two ratios are simple multiple of each other. This proves law of reciprocal proportions.