本文通过观测水滴半径随时间的变化,测定不同气压 P 下空气中水蒸汽的扩散系数 D,并由此可证气体分子运动论中所得到的理论关系,即在一定温度下,PD=常量.如果已知水蒸汽分子的平均速度 V,则可从D=1/3(?)·(?)求出水蒸汽分子在不同压强下的平均自由程(?). In this paper, by observing the variation of water droplet radius with time, the diffusion coefficient D of water vapor in air at different pressures P is measured, and the theoretical relationship obtained in gas molecular motion theory can be proved. That is, PD = constant at a certain temperature. If the average velocity V of water vapor molecules is known, the average free path (?) of water vapor molecules at different pressures can be found from D = 1/3(?)?(?).