XXIX. Photo-chemical researches.—Part III. Optical and chemical extinction of the chemical rays

In order to determine whether the act of photo-chemical combination necessitates the production of a certain amount of mechanical effect, for which an equivalent quantity of light is expended, or whether this phenomenon is dependent upon a restoration of equilibrium effected without any corresponding equivalent loss of light, we must study more specially the phenomena occurring at the bounding surfaces, and in the interior of a medium exposed to the chemically active rays. A certain large class of bodies permit the chemical rays to pass through them to a greater or less extent, whilst other substances are opake to these rays. The first class, to which the name “diactinous” maybe given, includes almost all colourless, blue, and violet media; the second class of “anactinous” bodies consists chiefly of opake, and yellow or red substances. Although the terms “diactinous” and “anactinous,” like “diathermanous,” “ diaphanous,” &c., merely represent phases of the same difference, and can therefore bear no strict scientific definition, yet we do not hesitate to employ these terms as a means of avoiding a tedious circumlocution.