How did we know about the existence of elements before quantum theory? [duplicate]








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I'm reading a book about the elements, and it says all the time: discovered/isolated by X in a year.


But how did the people know about the elements before the quantum theory? How did they know they were in front of a new "element"?





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> How did we know about the existence of elements before quantum theory?
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The short answer is that the old quantum theory had nothing to do with element discovery. Elements were considered elements after a thorough classical chemical analysis which included lots of gravimetry (weighing of precipitates), and observation of their novel chemical or physical properties. Phosphorus, for instance, was discovered by a German alchemist who collected buckets and buckets of urine in search of a recipe to make gold. One fine day, upon strongly heating dried urine residues strong with sand/coal he discovered a waxy substance which glowed in the dark for hours. Quantum theory had no part to play!


Modern textbooks teach science as a success story. In fact, science is also a story of many failures that nobody wants to record. Many elements are pre-historic like carbon, iron, copper.


When Bunsen and Kirchoff developed the spectroscope, it allowed them to see the spectrum of chemical compounds in a flame or in gas discharges. These new colors and patterns in the spectrum revealed brand-new elements. Cesium, rubidium, thallium and many gases were found this way. Helium was discovered in the Sun first! As before, no old quantum theory was involved, since the initial focus was on observation rather than determining the reason for things to happen. Even the electron and the proton were not needed to discover those elements. It was pure chemical analysis of newly discovered rocks and materials.


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