Law of reciporcal proportion.

The law of reciprocal proportion was proposed by Jeremias Ritcher in 1792.
It states that, “If two different elements combine separately with the same weight of a third element, the ratio of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine.”

Example: Oxygen and sulphur react with copper to give copper oxide and copper sulfide, respectively. Sulfur and oxygen also react with each other to form SO2. Therefore,
In CuS => Cu:S = 63.5:32
In CuO => Cu:O = 63.5:16

S:O = 32:16
S:O = 2:1

Now in SO2=> S:O = 32:32
S:O = 1:1

Thus the ratio between the two ratios is the following:
2/1:1/1 = 2:1

This is a simple multiple ratio.