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A colloid contains dispersed particles in a continuous medium. A fine particle suspension can form a colloid; the particles in the Perrin sedimentation experiment illustrate this case.

A monodisperse particle suspension has a narrow distribution of particle sizes. Treating its particles as having a single radius simplifies the Perrin sedimentation experiment model; residual size variation can still affect the inferred Avogadro constant.

Awarded to Jean Baptiste Perrin for the discontinuous structure of matter, especially sedimentation equilibrium. The Perrin sedimentation experiment describes the measurements.^[ref]

A colloidal grain in a liquid is large enough to see with a microscope, yet small enough for thermal motion to compete with its tendency to settle. In the Perrin sedimentation experiment, the equilibrium number density falls with height: a visible miniature atmosphere. Counting grains at different heights measures an energy scale; tracking their Brownian motion measures a transport rate. Connecting both to the same Avogadro constant makes the experiment a quantitative test of kinetic theory. The Wiener measure describes an idealized path law; these measurements instead test a physical model and its material parameters.

Jean Baptiste Perrin's 1909 memoir traces his sedimentation experiments to May 1908. It reports relative counts $100$, $47$, $22.6$, $12$ at heights $5$, $35$, $65$, $95\mu \mathrm{m}$, and compares radius estimates from Stokes settling velocity, counted grain mass, and rows of touching grains.^[ref]

The 1926 lecture describes resin precipitated from alcohol into water, washed and fractionated by centrifugation to obtain nearly monodisperse gamboge or mastic grains. Diameter and density were measured separately. A sealed slide-and-cover-glass chamber prevented evaporation; a vertical microscope resolved roughly $2\mu \mathrm{m}$ layers through a roughly $0.1\mathrm{mm}$ depth. Refocusing and photographing compared layer populations after equilibrium; changing temperature reversibly changed the distribution.^[ref]

In Les Atomes (1913), Jean Baptiste Perrin restricts the eyepiece view to fewer than five or six grains and alternates observations between heights. Focus displacement requires a refractive index correction to recover actual layer separation. Horizontal positions were marked with a camera lucida, usually every $30\mathrm{s}$, and distances calibrated with a micrometer; the polygon joins sampled positions rather than recording the intervening trajectory.^[ref][ref]

Sedimentation is separation of suspended particles through a body-force-driven relative motion. In the Perrin sedimentation experiment, diffusion balances gravitational settling to produce a stationary height profile.

Sedimentation equilibrium occurs when the downward tendency of suspended particles is balanced by their diffusion, leaving a stationary concentration profile with no net particle flux. For dilute identical particles in an isothermal fluid, let $m_b=({\rho }_p-{\rho }_f)V_p$ be their buoyant mass. Balancing the gradient of ideal osmotic pressure $n{k}_{B}T$ against gravity givesHere height increases upward, and heavy particles have $m_b>0$.

The profile is a miniature atmosphere: thermal motion spreads particles upward while gravity favours lower positions. The Perrin sedimentation experiment measured particle counts at different microscope focal heights and used this competition to infer a molecular energy scale.^[ref] A stationary profile requires sufficiently slow background flow and controlled temperature; a growing deposit during ongoing settling is a different situation.