Figure 7.1 — Brownian motion Figure 7.1 — Brownian motion. In 1908, Jean Perrin recorded individual trajectories of small (radius 0.53 μm) putty particles in water at 30-second intervals. In the three trajectories shown above, successive positions are joined by straight line segments. The observed random walk is due to collisions with water molecules. The mesh size is 3.2 μm. Adapted from Perrin (1909).