RECENT measurements of conduction1 and of ion transit time2,3 in liquid n-hexane have made use of the fact that electrons can be released into the liquid by the incidence of ultra-violet illuminations on the cathode surface. These measurements have been interpreted with the assumption that the ultra-violet light can be passed through the liquid to the cathode surface without producing ions in the liquid or otherwise disturbing it. Even in the most highly purified n-hexane it is not certain that this is true, particularly when subject to electric stress. The method described here attempts to overcome this difficulty.