Nowadays everyone knows that physical things -- whether solid, liquid, or gaseous -- are made up of atoms. The modern theory of atoms derives from the chemical work of John Dalton around 1800 (among other things, he tabulated the first table of the elements and was the first to assign different weights to the elements he identified). Yet the idea of atomism far predates Dalton and modern science. When it comes to atomic theory, everything after Dalton has been empirical, whereas everything before Dalton was speculative -- more philosophical than scientific and thus properly an "ism" rather than an account of how the world works.
Naturally, I find the pre-history to be interesting, in large measure because of its connection with Epicurus (I'll likely weave a bit of this into my Letters on Epicurus). The atomic hypothesis was originaly formulated by Leucippus and Democritus around 400 B.C., taken up at greater length by Epicurus around 300 B.C., and immortalized by Lucretius around 50 B.C. in his didactic poem On the Nature of Things. After that, the idea of atomism was lost in the West until the Renaissance. Giordano Bruno was an atomist of sorts, as was Galileo (at least in his Discourse on Floating Bodies of 1612). Francis Bacon, too, apparently flirted with atomism early in his career as a natural philosopher in the late 16th century. The most outspoken advocate for atomism in the early 17th century was Pierre Gassendi, who attempted to resurrect the philosophy of Epicurus and who tussled with Descartes over issues of both scientific method and physical theory (interestingly, Descartes opposed atomism on religious grounds: he held that God can't make anything so small that He can't divide it). Over the course of the later 17th and early 18th centuries, "Gassendism" fell out of favor in France, especially once it gained influence across the Channel in England through the work of Walter Charleton (Physiologia Epicuro-Gassendo-Charletonia, 1654) and others. Philosophically, that influence can be seen in Locke, Hume, and Reid among the British empiricists. Scientifically, Robert Boyle, who despite his alchemical interests is often credited as the founder of modern chemistry, held atomism or corpuscularism to be promising as a physical hypothesis; in The Sceptical Chymist (1661) he wrote:
I must advertize You, that I now mean by Elements, as those Chymists that speak plainest do by their Principles, certain Primitive and Simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the Ingredients of which all those call’d perfectly mixt Bodies are immediately compounded, and into which they are ultimately resolved....
Newton, too, adhered to atomism or corpuscularism (the latter, in a nod to the possible alchemical transformation of matter, held that different elements could be mixed together), and argued in his Opticks (1704) that light is composed of particles or corpuscles.
The traditional alternative to atomism was hylomorphism: the view that physical change consisted not in a rearrangement of atoms but in a transformation of a particular kind of matter into one of its potential states or forms. In particular, Aristotle rejected the idea of an actual void (necessary to account for physical change in early atomic theory) as an impossibility, and also raised questions about the supposed uniformity of atoms (e.g., why is it only atoms that can be uniform, but not larger entities?). Interestingly, modern atomic theory in a sense goes some of the way back toward Aristotle, since it shows that atoms themselves are in fact divisible. Further, Aristotle was most interested in the larger structures of especially living things (bones, sinews, veins, muscles, and other biological parts as well as animals themselves), not in simple entities like solids, liquids, and gasses. And, in any case, strictly speaking there was no evidence for atomism in the time of Democritus, Aristotle, Epicurus, or even Gassendi and Boyle: it was hypothetical, not yet scientically provable.
Despite its somewhat hazy and speculative beginnings, atomism turned out to be the more fruitful hypothesis in the end, and led rather directly to the insights of Boyle and Dalton, and to the rise of modern chemistry. Thus in the realm of physics and chemistry, too, the early views of Epicurus, Democritus, and others eventually played their part in the intellectual emancipation of mankind.