It was given by joseph Proust. in the year of 1774...in this chemical molecules always contain same elements and they r combined in small proportion by mass.....

The Law of Constant Proportions was given by Joseph Louis Proust, a French chemist, in the late 18th century. This law, also known as the Law of Definite Proportions, states that a given chemical compound always contains its constituent elements in fixed and definite proportions by mass.

Explanation:
The Law of Constant Proportions is based on the observation that different samples of a pure chemical compound always contain the same elements in the same proportion by mass. In other words, regardless of the source or method of preparation, the composition of a compound remains constant.

Proust's experiments:
Joseph Louis Proust conducted a series of experiments to support the Law of Constant Proportions. One of his notable experiments involved the analysis of copper carbonate, a compound composed of copper, carbon, and oxygen. Proust heated different samples of copper carbonate and found that, regardless of the amount of the compound used, the ratio of copper, carbon, and oxygen remained constant.

Significance of the Law:
Proust's Law of Constant Proportions was a significant contribution to the field of chemistry as it laid the foundation for the development of atomic theory. This law provided experimental evidence that compounds are made up of specific elements combined in fixed ratios, which supported the concept of atoms and their combining capacities.

Dalton's Atomic Theory:
John Dalton, an English chemist, further expanded on Proust's Law of Constant Proportions with his Atomic Theory. Dalton proposed that each element is composed of indivisible particles called atoms, and these atoms combine in simple whole number ratios to form compounds. Dalton's theory was consistent with Proust's law, as it explained the fixed ratios observed in compounds.

Conclusion:
In conclusion, Joseph Louis Proust gave the Law of Constant Proportions, which states that a given chemical compound always contains its constituent elements in fixed and definite proportions by mass. His experiments and observations provided evidence for the existence of fixed ratios in compounds, supporting the development of atomic theory.