A search was made for liquid mixtures giving low expansion ratio, low background fog, and good electron tracks in cloud chambers filled with air at near atmospheric pressure. As was expected from theory, mixtures of any lower alcohol with water gave a minimum in the curve of expansion ratioversusconcentration, while no minimum was found in the curve for mixtures of any two alcohols. The optimum mixture found for chamber use was 50 percent ethyl or normal propyl alcohol, 25 percent water, and 25 percent acetone; the presence of acetone definitely increases the contrast between tracks and background fog. A brief study of photochemical fog, with a quartz mercury arc, showed that wave‐lengths below 3900A are chiefly responsible for such fog. Irradiation of pure alcohols gives no perceptible fog, but irradiation of the optimum mixture above produces dense fog. However the clean‐up after irradiation is sufficiently rapid to offset the disadvantage of its sensitivity to light.