## Abstract

An accurate method is described for determining the differential stopping power of liquid water at $\alpha $-particle energies between 4 and 6 MeV. $\alpha $-Particles from a Th C$^{\prime}$ source pass through a steel capillary tube, whose covered end lies just below the water surface in a wide glass tank. After traversing the water film above the capillary tube the $\alpha $-particles are detected by a movable proportional counter. The curve of count against distance is first plotted. Then the thickness of the water film is increased by adding a weighed quantity of water to the tank and the curve is plotted again. The air equivalent of the added layer of water is then derived from the displacement of the curve. A source of error in the measurements is hysteresis of the angle of contact of the water with the walls of the glass tank, but a correction may be derived by varying the length of the water-glass-air boundary of the system, or the error may be eliminated by using dilute solutions of detergents. At an $\alpha $-particle energy of 5$\cdot $5 MeV, the differential stopping power of one molecule of water relative to the average atom of air is found to be 1$\cdot $45 $\pm $ 0$\cdot $02, which is 0$\cdot $02 less than the accepted value for an equivalent mixture of hydrogen and oxygen. The stopping power does not increase by more than 0$\cdot $02 $\pm $ 0$\cdot $015 between 5$\cdot $7 and 4$\cdot $3 MeV.