= Perrin sedimentation experiment
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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 $5$, $35$, $65$, $95\,\mathrm{\mu m}$, and compares radius estimates from <Stokes settling velocity>, counted grain <mass>, and rows of touching grains.<Mouvement brownien et réalité moléculaire by Jean Baptiste Perrin>[1909 memoir, §§15, 20–22; scan pp. 28, 41–49]{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 $2\,\mathrm{\mu m}$ layers through a roughly $0.1\,\mathrm{mm}$ depth. Refocusing and photographing compared layer populations after <equilibrium>; changing <temperature> reversibly changed the distribution.<Nobel Lectures, Physics 1922–1941/Discontinuous Structure of Matter by Jean Baptiste Perrin>[Nobel Lectures, Physics 1922–1941; Discontinuous structure of matter, 11 December 1926, pp. 148–150]{ref}
In \i[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.https://fr.wikisource.org/wiki/Les_Atomes/III[Les Atomes, chapter III, §§61–64]{ref}https://fr.wikisource.org/wiki/Les_Atomes/IV[Les Atomes, chapter IV, §§70–75]{ref}
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