Viscosity — the resistance of a fluid to flow — is among the most important physical properties in chemical and mechanical engineering. This experiment uses an Ostwald U-tube viscometer, in which the time for a fixed volume of liquid to flow through a precision capillary under gravity is measured for both the test liquid and distilled water (the reference). Poiseuille's law provides the theoretical basis: from the ratio of flow times and densities, the relative viscosity is obtained without any instrument constant: eta1/eta2 = (rho1 × t1)/(rho2 × t2). If the absolute viscosity of water at the measurement temperature is known (0.890 cP at 25°C), the absolute viscosity of the unknown liquid follows directly. The chapter details the critical procedural requirements: perfectly vertical mounting of the viscometer, constant temperature, use of a suction bulb (never mouth suction), and thorough cleaning with chromic acid then acetone. A worked example with glycerol illustrates calculation and result interpretation, with relative viscosity approximately 1165 times that of water at 25°C.