Photo-chemistry / by S.E. Sheppard.

  • Samuel Edward Sheppard
Date:
1914
Catalogue details

Licence: In copyright

Credit: Photo-chemistry / by S.E. Sheppard. Source: Wellcome Collection.

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    absorption of the substance for light. As a first approximation it may be written— K = ~ * • x) where rn is the ordinary refractivity for supposed minimum absorption, and k is the absorption index, which is a function of the wave-length or vibration-frequency of light {vide p. 136). The symbol i = V — 1, and is not the symbol of a numerical quantity but of an operation.1 The relation of the refractivity and dispersion of light by media to their chemical constitution nd to the dynamics of the absorption of light is discussed later. x § 13. Photometry. On considering a chemical system acted on by light, it is evident that the extent and progress of the action will depend on a large number of purely physical factors, some peculiar to light as a phenomenon independent of physical bodies, others depending on its interaction with these. We require to know (1a) the amount of energy radiated in relation to the nature, size, form, etc., of the source; (b) the light-energy incident on the chemical system, which, again, depends on such factors as its position, its reflecting power, and so forth; (c) the energy transmitted. Now, from what has already been said, it is obvious that we can make measurements of the radiated energy in three ways. I. As light, by physiological action. II. As heat, by which we can measure the total energy. III. By chemical action. 'These three methods correspond to the titles (I.) Photo- metry, (II.) Radiometry, (III.) Actinometry. Now, as far as the physical factors indicated above are con- cerned, the laws are the same, and all the conditions are equally applicable, although they were historically derived by the methods of photometry in the narrow sense. We shall now briefly consider the more important relations, and the 1 Algebraical expressions in which the symbol sj — 1 occurs are termed “ imaginary ” or “ absurd ” quantities. Cf. A. N. Whitehead, An Intro- duction to Mathematics (Williams and Norgale, 1911).
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