Which law of chemical combination?

Chemistry Level pending

1.08 gram of $$\ce{Cu}$$ wire was allowed to react with nitric acid. The resulting solution was dried and ignited when 1.35 gram of copper oxide was obtained. In another experiment 2.3 gram of copper oxide was heated in presence of Hydrogen yielding 1.84 gram of copper.The above data is in accordance with the law of $$\text{__________}$$.

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