From In Search of Schrödinger’s Cat: Quantum Physics and Reality
During the 1860s and 1870s these pioneers developed the idea that a gas is made up of very many atoms or molecules (the number derived from Avogadro’s hypothesis gives you some idea how many), which can be thought of as tiny, hard spheres that bounce around, colliding with one another and with the walls of the container that holds the gas. This related directly to the idea that heat is a form of motion — when a gas is heated, the molecules move faster, which increases the pressure on the walls of the container, and if the walls are not fixed in place, the gas will expand. The key feature of these new ideas was that the behavior of a gas could be explained by applying the laws of mechanics — Newton’s laws — in a statistical sense to a very large number of atoms or molecules. Any one molecule might be moving in any direction in the gas at any time, but the combined effect of many molecules colliding with the walls of the container each second produces a steady pressure. This led to the development of a mathematical description of gas processes called statistical mechanics. But still there was no direct proof that atoms existed; some leading physicists of the time argued strongly against the atomic hypothesis, and even in the 1890s [Ludwig] Boltzmann felt himself (perhaps mistakenly) to be an individual struggling against the tide of scientific opinion. In 1898, he published his detailed calculations in the hope “that, when the theory of gases is again revived, not too much will have to be rediscovered”; in 1906, ill and depressed, unhappy about the continuing opposition of many leading scientists to this kinetic theory of gases, he killed himself, unaware that a few months before an obscure theorist called Albert Einstein had published a paper that established the reality of atoms beyond reasonable doubt.
