Actinism, the chemical principle of light. Three distinct principles emanate from the sun -- light, heat, and actinism. Numerous examples of the effects of their influence occur daily, which are erroneously attributed to the light which we see. It is actinism which fades colors, bleaches linen, rots fabrics, tans human skin, puts out the fire, and performs the operations of photography. It acts principally by abstracting oxygen from the bodies which it affects. There may be actinism without light, or vice versa. Yellow glass transmits the latter, but stops the former. Dark blue glass, which transmits but little light, is quite pervious to actinism. Blue objects reflect great quanitities of it, while red or yellow ones reflect but little or none. The electric and lime lights give out great quanititism from their blue tinge; and gas and candles but very little, from their yellow color. The amount of actinism received from the sun differs considerably, according to the time of year, being at its maximum about the end of March, and gradually diminishing until the end of December, when it arrives at its minimum. Actinism, in large quantities, is necessary to the proper condition of the human system.

Entry from Everybody's Cyclopedia, 1912.