At a simple level, matter can be thought of as atoms that are indivisible, so long as we know that this is a very simple first approximation to the whole truth.
Atoms cannot be created or destroyed in theory, but in practice, many atoms can be changed permanently, in small numbers, by interactions with their nuclei.
Atoms have characteristics which can be measured, such as having a measurable size and they have a constant mass that can be measured with a mass spectrometer.
Atoms may not be seen, but the positions of individual atoms may be located in a variety of ways, increasing our confidence that atoms are real objects.
Some time before 50 BC, the poet Lucretius had suggested in Rome that matter was made of atoms, though these atoms were little like the atoms we know today.
In 1808, John Dalton published his theory that all matter was made of atoms, bringing a revolution to chemistry, even though others had suggested atoms earlier.
John Dalton's first principle in his atomic theory was that the chemical elements are atoms which do not change, even when they take part in a chemical change.
John Dalton's second principle, given that the elements are made of unchangeable atoms was that all of the atoms of a particular element are identical.
John Dalton's third principle in his atomic theory , given that atoms exist, was that chemical compounds form when atoms combine in simple numerical ratios.
Under some circumstances, the indivisible atoms may be considered in terms of their components to any degree of complexity, depending on the detail we need.
For most parts of chemistry it is sufficient to consider atoms to be made up of protons and neutrons in the nucleus, and shells of electrons orbiting around it.
An atom's emission spectrum reflects quantisation in a way that we can observe in our less confusing real world where most quantum effects are hidden from view.
Sir William Crookes used spectral analysis to discover the element thallium compounds as an impurity in selenium ores, though he did not isolate the element.
Jean Foucault, the inventor of the pendulum, probably also first discovered the way the emission and absorption effects are linked, but the did not publish it.
The absorption spectrums of atoms may also be taken as evidence that atoms are real objects, rather than theoretical constructs dreamed up by theoreticians.
A laser mass spectrometer can identify tiny samples by molecular weight, after the molecules are fragmented and accelerated so their momentum can be measured.
The observation of Brownian motion provides direct evidence for the existence of atoms as small particles in a colloid or a suspension are seen to be buffeted.
Diffusion happens when atoms or molecules move randomly. It offers further evidence for matter existing as atoms and molecules since light gases diffuse faster.
A mass spectrometer 'weighs' atoms, and the fact that it gives constant results, allowing for isotopes, offers further evidence that matter is made up of atoms.
In 1799, Proust showed that copper carbonate from several sources had the same amounts of copper, carbon and oxygen, leading to the Law of Constant Proportions.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsatoms.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.