Photo-Electricity #250781

It may seem somewhat strange that ionisation-by ultra-violet light manifests itself as a surface effect in solids and liquids, while ionisation by almost all other agents appears most evident in gases. Yet there is no fundamental distinction on this account. Ultra-violet light — provided extremely Short wave-lengths are used — does ionise gases just as readily as X-rays. But with the ordinary sources Of ultra-violet light, the surface effect is the only one Observed, and the methods, Of investigating it differ considerably from the methods employed in studying gaseous ionisation.