THE ACTION OF LIGHT 551 



two-thirds of its intensity by passing through a stratum of a particular solution of one centi- 

 metre thickness. Then, 



e = - log |=log 3 - log 2 = 0-176091. 



o 



By Beer's law, the absorption of light by solutions is directly proportional to 

 the concentration ; so that, if we know the extinction coefficient for a known 

 concentration, we can estimate an unknown concentration by measuring its 

 extinction coefficient. Hence the practical value of the form given to the 

 extinction coefficient. Thus : 



c and c being the respective concentrations, and 

 e and e' the respective extinction coefficients, 



Such measurements have played a large part in the investigation of the blood 

 pigments. 



Resonance. Light consists of a series of periodic electro-magnetic disturbances 

 of various periods of vibration, or wave length. We have seen (page 88) that 

 when a system, such as a pendulum, has the same period of vibration as that of a 

 series of minute impulses delivered to it, the system is set into vigorous movement 

 by the heaping up of the effect of a number of small impulses. It is, in fact, a 

 means of accumulating energy. Consider now the effect of a set of various wave 

 lengths, such as we find in the sun's light, on a chemical molecule, which has itself 

 a definite rate of vibration. Some of the rates of vibration of the different light 

 waves will almost certainly coincide with that of the molecules of the absorbing 

 substance and will therefore set these into resonant vibration, which may reach an 

 amplitude great enough to bring about chemical change. At the same time, those 

 rays of the vibration period in question will be absorbed and, if situated in the 

 visible part of the spectrum, there will be an absorption band seen by the eye. If 

 in the ultra-violet, as in the case of many colourless organic compounds, the band, 

 although invisible, may be photographed. 



Spectrophotometry. Observations by the spectroscope give us information of 

 the position of absorption bands, but we often require measurements of the degree 

 of absorption by different substances in various regions of the spectrum. This is 

 done by the method of Spectrophotometry, based on the laws of Lambert and Beer. 



In practice it consists in the comparison of the intensity of the light of a 

 particular wave length, which has passed through a known thickness of the solution 

 investigated, with light of the same wave length which can be diminished in 

 intensity in a known degree. 



The practical methods of doing this and of calculating the extinction coefficients will be 

 found in Gamgee's article (1898, pp. 213-225). The price lists issued by Messrs Adam Hilger 

 are instructive, especially with regard to the beautiful instruments made by them for the 

 registration, photographic or otherwise, of spectrophotometric measurements. Fig. 176 

 (page 549) is a copy of the curve of the ultra-violet absorption of anthracene as obtained by 

 one of these instruments. The paper by Eckert and Pummerer (1914) may also be consulted 

 with respect to photographic registration. 



GENERAL THEORY OF PHOTO-CHEMICAL ACTION 



We may take it then that light of some particular wave length is absorbed, and 

 that it sets into resonant vibration the molecules of the absorbing substance, if any 

 of the vibration periods of the light waves coincide with those of the latter. 

 What is the further course of events ? We know that, in many cases, chemical 

 reaction follows. 



Let us see first what are the general phenomena with which we have to deal. 

 The article by Luther (1908) may be referred to for more details of the general 

 theory than can be given here, and the monographs by Weigert (1911) and 

 Sheppard (1914) for the whole subject. 



