Early 19th-century chemist John Dalton (1766-1844) made several important if partially obsolete contributions to the essential early development of atomic theory. According to his theory, all elements are made up of fundamentally different atoms, with measurably different weights; the atoms of different elements can combine to create chemical compounds; and these atoms cannot be created or destroyed, merely grouped together in new ways via chemical reactions.
Like many early scientists, Dalton was more a Renaissance man than a chemist per se, conducting memorable research not only in chemistry but in meteorology and physics, as well as into colour blindness (which he suffered from, himself). Born a Quaker weaver's son, Dalton's religious faith meant he was legally barred from an education in law or medicine, but he was able to win a teaching position at a Manchester academy through the assistance of philosopher John Gough. His first works of note, during the 1790s, were actually not in chemistry, but in meteorology (Meteorological Observations and Essays), English grammar, and colour blindness, the latter of which came as a member of the Manchester Literary and Philosophical Society.
Dalton became secretary of the Society in 1800 and used this position to push forward new discoveries, published in the organization's journal several years later. In a series of four "Experimental Essays," he elaborated a new chemical theory in four parts: gases; steam and vapour pressure; evaporation; and thermal expansion. His most important later discoveries about the makeup of the atom, therefore, actually proceeded from important but seemingly minor inquiries into the nature of steam pressure, then used to power the Industrial Revolution.
DALTON'S ATOMIC THEORY
Dalton's theory, as espoused in the essays, consisted of five main points. First, the material world is made up of elements, and all of these elements consist of atoms, extraordinarily miniscule particles which cannot be seen by the naked eye or (at least at that time) under a microscope. Second, these atoms can neither be created nor destroyed by human intervention, nor can they be subdivided. Note, here, the similarity to Newton's theory of energy; in point of fact, as we now realize, atoms can be both created and destroyed, through nuclear fission and nuclear fusion.
Third, according to Dalton, all of the atoms of the same element will be identical. Moreover (and fourth), these atoms are unique: other elements have entirely different atoms. This is more or less true according to modern chemistry - we now know that atoms of the same element may differ in their number of neutrons (and, if bonded or ionized, their number of neutrons), but not their number of protons.
Fifth, and finally, Dalton predicted that atoms of different elements could combine together in chemical reactions and form new chemicals, called compounds. Like their constituent atoms, these compounds would always be of a predictable and regular type: for example, water (H2O) will always contain two hydrogen atoms and one oxygen atom. This important insight now proceeds from our understanding of chemical bonding, but to Dalton, it was a logical deduction based on his earlier observation that atoms were neither created nor destroyed: since they must still be present after chemical reactions had occurred, the atoms must have (and, indeed, had) simply rearranged themselves into new groups with new properties.
Given the comparatively crude scientific instruments of his time, it is obvious that many of Dalton's insights were more educated guesses than rational deductions. Nevertheless, they have generally stood the test of time. That atoms can, in fact, be created and destroyed is a twentieth-century realization following the discovery of radioactivity and the application of nuclear fission. Moreover, atoms of different elements are now known to have different atomic weights, due to the existence of different isotopes, although they do always contain the same number (and weight) of protons. Nevertheless, these observations form the basis of early fundamentals of chemistry and, for the most part, are still broadly accurate if rough estimates of chemical reality.