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Read · Brownian motion: from wandering to a measurable law · Results
Particles just large enough to see under a microscope must wander, Einstein argues, and how far they wander in a given time would let us count molecules. Read the argument, and open any step it leaves out.
This is newly written explanation in modern notation, and its editorial review is pending. It is not the German source, an English translation, or a complete edition of the paper. The German source face holds a machine-drafted transcription with hand correction that no one has reviewed yet, and the facsimile face shows the pinned journal pages; an English translation is not ready. The headings name the part of the argument each passage discusses; they are not a list of the paper’s paragraphs.
Classical thermodynamics, as Einstein characterizes it, expects no force on the wall from suspended bodies, because the free energy seems not to depend on where they are. On the molecular-kinetic view a dissolved molecule differs from a suspended body only in size, so equally many of either, far enough apart, should exert the same osmotic pressure, RT/N times their number per unit volume.