The apparatus used to show that aluminium was opaque to the ions from a flame consisted (see fig. 1) of a tube A, which was

funnel-shaped at one end, the other end of which led to an enclosure B, one side of which (a b) was made of aluminium foil connected to earth, and on the other side there was an insulated metallic plate (c d), which could either be connected to earth or to a high potential battery. On the other side of the aluminium foil there was an electroscope in a closed case C. The electroscope was of the type used by C. T. R. Wilson in his experiments on the natural ionisation of air in closed vessels.