How did Joseph Proust measure mass composition of NATURALLY occurring compounds?

note: I am not a chemist, I am just interested in this


Proust famously compared the mass composition by element of natural and artificially prepared copper carbonate and concluded they were the same, but-


1. Since this is early atomic hypothesis days, how did he know to compare Cu, O and C and not Cu and CO2 or something ie. how did he know what the elements were?
2. I assume he determined mass composition of the artificial carbonate by measuring the elements he used to prepare them (a equation of what he did would be helpful), but how did he measure composition of the naturally occurring carbonates, don't the reactions for their breakdown leave compounds behind that are not easy to decompose to elements, and then he had to somehow separate and weigh them all


Any detailed account of his experiments would be helpful




For Proust and all chemists of this time, an element is a substance that nobody has been able to decompose into more fundamental constituents, whatever the method used (heating, electrolysis, etc.) For example, the copper carbonate that ThatApollo777 proposes, is not an element: it can be decomposed into two simpler substances, by heating: copper chalk (today copper oxide) and fixed air (today $\ce{CO2}$). Of course copper chalk is not yet an element, as it can be reduced (its mass is reduced) by using a reducing agent like flammable air (today Hydrogen $\ce{H2}$). This gives copper (which is an element) and water. Now water is not yet an element, as it can be decomposed by electrolysis into two substances that was shown to be elements: Hydrogen and Oxygen.


All these statements have been difficult to state and many times reexamined. This made endless discussions in the scientific world in the beginning $19$th century, which cannot be explained here in detail: it would last too long, and it is often boring.


Some generalities were sometimes discovered at Proust's times. For example, that metals are in general elements. The reason is that metals cannot be decomposed, whatever the attempts carried out to do it. On the other hand, some elements sometimes were not considered as such. For example, Chlorine $\ce{Cl}$ was for many years considered as an oxide (and not as an element), because all known salts in the years 1800 were made of at least three elements: one metal, one non-metal and oxygen. Of course, the main salt (today $\ce{NaCl}$) refused to be spliced into three parts. This failure was attributed to a lack of valid experimental techniques: chlorine $\ce{Cl2}$ was a long time considered as a compound of oxygen plus an unknown non-metal, to be discovered some day. Alas, it was never discovered. And chlorine stayed in the already long family of elements!


So as you see, there is no simple way of stating that a substance is an element or a compound, at Proust's time. Today the problem does not exist anymore, because atoms are known. Atoms were not really accepted at Proust's time. Their existence was only considered as an hypothesis. And there were other hypothesis in chemistry.


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