In form energia. The capacity of light to cause chemical change; = actinism, n. 1. Obsolete.

In the terminology of John William Draper: the photochemical changes caused by exposure to tithonic rays, esp. those on a photographic plate. Cf. tith…

The property by which light or other electromagnetic radiation causes chemical change, as in traditional photography and in photosynthesis. Also in…

= actinism, n. 1. rare.