Dalton's Atomic Theory
Today, John Dalton is one of the most well known scientists from the 1800's. In addition to proposing an accurate three-part atomic theory, he also discovered colour blindness and made many important observations about the characteristics of weather.
Less than twenty years before the establishment of Dalton's theory, Lavoisier focused on making careful quantitative measurements, which allowed him to determine the composition of compounds with amazing accuracy. By 1799 enough data had been accumulated for Proust to state the Law of Constant Composition, and by 1803 Dalton began his work with oxygen and carbon, noting that it combined to make two compounds. Each compound had its own weight ratio for oxygen in relation to carbon, but for the same amount of carbon, one compound had had exactly twice as much oxygen as the other. From these and other findings, Dalton proposed the Law of Multiple Proportions, which was later verified by Berzelius.
Dalton formed his complete atomic theory in attempt to explain how and why elements combine in fixed ratios or multiples of those ratios. The theory stated that elements were made up of tiny particles called atoms, and the reason that elements were pure was because all of the atoms in a particular element were identical, down to the same mass. Dalton also stated that the reason elements are different from one another is because the atoms in each of the elements are different, in particular, the have different masses. The theory also states that compounds consist of atoms of different elements combined together, but they are pure because these atoms are bonded to each other (possibly by hooks), and aren't easily separated. Dalton also recognized that compounds have a definite composition because they have a fixed ratio of atoms, each with its own weight.