Actinism

Actinism (Gr. aktis, 'a ray of sunlight') is a term whose signification has varied considerably. It seems to have been invented by Sir J. Herschel (about 1824), and with him it was used to express the heating power of sunlight. His Actinometer was employed for the same purposes as the Pyrheliometer (q.v.). Later, the term was applied to the property—which certain rays of light (alone) appeared to possess—of decomposing chemical compounds (see SPECTRUM and PHOTOGRAPHY). Recent discovery has proved that there is no special class of actinic rays, it being found possible to utilise any of them photographically by proper physical and chemical processes. The general treatment of the subject will be found under Radiation of Heat; and the term Actinism, unless a perfectly novel sense be invented or discovered for it, is now superfluous.