162 BIOPHYSICALLY ACTIVE LIGHT 



to produce death, it follows that about three million photons are 

 necessary to cause death. 



The lethal action of the ultraviolet energy is not brought about by a 

 chemical change of the medium surrounding the organism. The funda- 

 mental reaction which causes death is produced inside the cell by the 

 radiant energy that can penetrate to this depth. The reaction is prob- 

 ably unimolecular, on the basis of the assumption that the number of 

 bacteria killed should be proportional to the radiant energy intercepted 

 by a critical volume in the organism. 



Rentschler [1940] found that the bactericidal action is determined by 

 the amount of radiant energy to which the bacterium is exposed, regard- 

 less of whether a high intensity is applied for a short time or a low 

 intensity is applied for a correspondingly long time, provided that the 

 product, intensity X time, is the same. 



The lethal action is independent of the temperature of the bacteria 

 at the time of exposure. 



The sensitivity of an organism to ultraviolet radiation varies appre- 

 ciably at different stages of its life cycle. It has been found that younger 

 cells are more sensitive than older cells to all forms of absorbed ionizing 

 radiations. 



Ultraviolet Activation 



In 1924 Hess and Steenbock independently and almost simultaneously 

 announced the discovery that exposure of edible materials to ultraviolet 

 light endows them with antirachitic activity. It has developed that this 

 activation is relatively permanent and that it is not a process of 

 oxidation. 



The second stage in the development was reached when it was demon- 

 strated that sterols became antirachitic upon irradiation. With the 

 introduction of the quartz spectrograph for investigating the spectral 

 absorption of the material under examination the problem entered its 

 quantitative phase. 



It was found that in foodstuffs the sterol fraction contained the 

 " acceptor " of the activating rays. The trend of the investigation then 

 turned to the solution of the chemical changes induced by the radiation 

 in the sterols. It was found that ordinary cholesterol was somewhat 

 opaque to ultraviolet light and that irradiation decreased its opacity. 



Schultz and Morse, working with cod-liver oil in 1925, found that the 

 absorption spectrum of ordinary cholesterol contained a definite band 

 structure, with maxima of absorption at about 2940 and 2830 A. The 

 bands disappeared after irradiation and only general absorption 

 remained. They assumed that the cholesterol was contaminated by a 



