1120 BIOLOGICAL EFFECTS OF RADIATION 



Laurens (95), Pincussen (120), Raiim (126), Reichel (127), Weinstein 

 (157), Wiesner (160), and Winterstein (162). 



SUNLIGHT 



It is generally granted that the observations of Downes and Blount 

 (35), published in 1877, constitute the real "discovery" of the killing 

 action of light on bacteria and other microorganisms, thus drawing the 

 attention of biologists to the significance of the chemically active spectral 

 radiation. They showed primarily that certain organic products in 

 solution, undergoing decomposition and decay by the action of a mixture 

 of bacteria, would be inhibited in this decomposition by long exposure 

 to direct sunlight. Fresh extracts thus exposed were largely prevented 

 from the usual course of decomposition. They recognized the importance 

 of determining the regions of the spectrum involved and the need of 

 intensive study of this effect. There followed almost immediately 

 numerous qualitative observations of interest, mostly with liquid cul- 

 tures, all serving to indicate that the lethal effects of sunlight were not 

 limited to a few sensitive organisms, and that there was a definite need 

 of recognizing in sunlight a "new" factor in the environment of micro- 

 organisms. The sanitation aspect and the "natural purification" of the 

 water of rivers were practical applications stimulating investigation. 



Leading students of bacteriology took the time to invade this field of 

 observation, bringing to bear upon it improved bacteriological technique. 

 In several short papers Duclaux (39, 41) reported killing effects even 

 upon spores (cf., also, Straus, 145), and he observed some diversity in 

 the resistance to light of the several organisms studied. From a practical 

 standpoint he regarded light as a universal disinfectant. The work of 

 Arloing (4, 5) on the anthrax bacillus was confirmatory of the earlier 

 studies, and he was particularly struck by the fact that anthrax spores, 

 known to be highly resistant to high temperature, are easily destroyed by 

 sunlight as well as by light from certain artificial sources. He ascribed 

 bactericidal action to the entire sun spectrum. Roux (133) concluded 

 that oxygen might be a factor in the lethal action of sunlight, a considera- 

 tion frequently revived in some of the later work. Tizzoni and Cattani 

 (149) demonstrated the lethal effect of sunlight on the vegetative cells 

 and spores of the tetanus bacillus. 



Since many of the experiments made at this time were on animal or 

 human parasitic organisms, and since all such organisms were found 

 sensitive to light, overemphasis was placed on the idea that parasitic 

 organisms exhibited a unique relation to this factor of the environment. 

 Extravagant claims were made regarding the disinfecting value of even 

 diffuse light of low intensity, and such extreme views were not overcome 

 for a period of years. On the other hand, some very careful experimental 

 work was done, and in this connection reference should be made to a 



