The nucleation rate of droplets from water vapor in a helium atmosphere was measured with an expansion cloud chamber as a function of temperature, supersaturation, and sensitive time. In addition to homogeneous nucleation, a form of heterogeneous nucleation was observed to occur above the ion limit, presumably upon neutral centers whose concentration is low and seemingly dependent upon the initial partial pressure of water vapor present. The semiphenomenological classical theory was modified to take into account the change in the free energy of formation of the critical cluster brought about by the presence of a single foreign molecule. The temperature dependence of these data closely confirms that found by a number of earlier investigators. The form of heterogeneous nucleation observed here makes the disparity in the temperature dependence of the nucleation rate of previous work understandable. As higher and higher nucleation rates are sampled, the centers upon which heterogeneous nucleation occurs are depleted and homogeneous nucleation becomes dominant. It was definitely established that the nucleation rate of water vapor is higher in an argon atmosphere than in a helium atmosphere. The latter result seems to indicate that the noncondensable gas plays a role in the clustering process and may possibly hydrate under appropriate circumstances. © 1969.
