The sixteenth century saw some foundations of modern chemistry being laid as people learned how to extract metals from ore and how to use some minerals for medicinal applications. We should probably call Robert Boyle the first true chemist as he learned how to perform quantitative experiments and discovered the relationship between pressure and volume of air. Boyle also began to develop better concepts about the elements. He viewed a substance to be an element if it could not be broken down into two or more simpler substances. This led to the death of the original list of four elements.
It was not until the eighteenth century that combustion was finally understood based upon the experiments of Lavoisier who discovered that oxygen was involved in combustion and also that life was supported by a process which involved oxygen. It observations led to his discovery of the Law of Conservation of Mass: Mass can be neither created nor destroyed..
In the early 1800's, the chemist Proust discovered what is now known as theLaw of Definite Proportion: A given compound always contains the exactly the same proportion of elements by mass.. This discovery led Dalton to consider that atoms might be the individual particles that make up the elements. Dalton noted that carbon and oxygen formed two different compounds with different properties and different masses -- one of them contains exactly twice as much oxygen as the other. This principle was found to apply to other elements as well and led to the Law of Multiple Proportions: When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always to reduced to small whole numbers..
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