The design of an improved version of the upward thermal diffusion cloud chamber is described. Such a chamber was constructed and used to measure the supersaturations required to observe a rate of homogeneous nucleation of 2-3 drops cm-3 ·sec-1 Measurements were made on benzene, toluene, orrho -xylene, and n -butylbenzene over as much as a 150'C range of temperatures. The results obtained are compared to the predictions of the classical theory of homogeneous nucleation (Volmer-Becker-Doring-Zeldovich) and of the Lothe-Pound theory. The classical theory is found to be in excellent agreement with the experimental results. an improved version of the diffusion cloud chamber Katz 18 (in the first paper of this series, which we hereafter refer to as I) found that, for n-hexane, n-heptane, n-octane, and n-nonane over the 100 qc range of temperatures studied, the measured supersaturations which caused homogeneous nucleation were very accurately predicted by the classical theory.At about the same time Wegener 19 began studies of homogeneous nucleation in a supersonic nozzle. He and his co-workers obtained good agreement with the classical theory for water 20 and for ethanol. 21 In view of this agreement of results with each other and with theory by various methods and by different workers, one might 448