A Simplified Atomic Theory Timeline (pre-1910)

It is easily accepted today that matter consist of atoms. At least according to middle school curricula, atoms are taught to be very small, almost indestructible particles whose combinations and interactions give rise to properties of materials. Such knowledge, granted almost effortlessly without context of its history, combined with rote learning often results in underwhelmed students overlooking the significance of such simple fact. Students often felt, myself in the past included, that the concept of atoms belongs to the realm of textbooks and classrooms – it will be irrelevant to post examination lives. 

We must consider the incredible fact that atoms are extremely small. They are at least a thousand times shorter than the shortest wavelength of visible light; so small that we could arrange a million of them to the breadth of a human hair. Unlike the curvature of earth, atoms are so small; no one has ever actually seen it. Even the most powerful microscopes today can only catch them as smeared “bumps”. As you can imagine, most evidences for its existence has to be extremely subtle. It is not surprising scientist took literally a century of different experiments, carried out at different time with different people, of which results have no simple means of explanation except adopting the existence of corpuscular matter as part of reality.

List of experiments and scientific discussions (non-exhaustive) that require the existence of corpuscular matter or explained through atomic structure (up to early 1900s):

1. On The Absorption of Gases by Water and Other Liquids, J. Dalton (1805)
2. Note on the Relative Masses of Elementary Molecules, or Suggested Densities of Their Gases, and on the Constituents of Some of Their Compounds, As a Follow-up to the Essay on the Same Subject, A. C. Avogadro (1811)
3. Brownian Motion, R. Brown and contemporaries (1785-1827), explained by A. Einstein (1905)
4. Kinetic Theory of Gases, M. Lomonosov (1747), G. Le Sage (1780), J. Herapath (1816), J. J. Waterston (1843), A. Kronig (1856)
5. Nature of Cathode Rays, J. J. Thompson (1897)
6. Geiger-Marsden Experiment, E. Rutherford (1909)

Not mentioning discovery of isotopes, neutron, etc. in later years.

By the end of the twentieth century, you can say we were really forced to believe atoms exist without actually touching or seeing one. Since no single piece of evidence is by itself a complete description of an atom, we can say the atom is a philosophical construct that has been refined through theories and tests that continues to this day.