What is constant proportion

A molecule of water will always have two hydrogen atoms and one oxygen atom, meaning that in a bottle of pure water the hydrogen to oxygen atom ratio will always be 2:1. This seems very logical to us today. But 300 years ago, scientists didn't understand compounds this way. However, in 1794, Joseph Proust published the Law of Constant Proportions, which says that a chemical molecule will always contains the same elements in the same proportion. There are two parts to the law of constant proportions: 1) there will always be the same elements that make up a compound, and 2) the mass of these compounds will always be in the same proportion. Elements in Compounds Pure water will always have hydrogen and oxygen atoms. It will never contain nitrogen, carbon, or phosphorus; it will only have hydrogen and oxygen atoms. Pure glucose will only contain carbon, hydrogen, and oxygen but no other elements. Pure ammonia will only contain nitrogen and hydrogen.