EFFECT OF LIGHT 19 



and found that sunlight inhibited the growth teuipoiarily. A large 

 number of experimenters on the Continent and in England have 

 worked at this fascinating subject since 1877, and though many of 

 their results appear contradictory, we may be satisfied in adopting 

 the following conclusions respecting the matter : — 



(1) Sunlight has a deleterious effect upon bacteria, and to a less 

 extent on their spores. 



(2) This inimical effect can be produced by light irrespectively of 

 rise in temperature. 



(3) The ultra-violet rays are the most bactericidal, and the 

 infra-red the least so, which indicates that the phenomenon is due 

 to chemical action. 



(4) The presence of oxygen and moisture greatly increase this 

 action, the process being largely an oxidation. 



(5) Sunlight also acts prejudicially upon the culture medium, 

 and thereby exerts an injurious action on the culture. 



(6) The time occupied in the bactericidal action depends 

 upon the intensity of the light and the inherent vitality of the 

 organism. 



(7) With regard to the action of light upon pathogenic organisms, 

 some results have recently been obtained with Bacillus typhosus. 

 Janowski maintains that direct sunlight exerts a distinctly depressing 

 effect on typhoid bacilli. At present more cannot be said than that 

 sunhght and fresh air are two of the most powerful agents we possess 

 with which to combat pathogenic germs. 



A very simple method of demonstrating the influence of light 

 is to grow a pure culture in a favourable medium, either in a test- 

 tube or upon a glass plate, and then cover the whole with black 

 paper or cloth. A little window may then be cut in the protec- 

 tive covering, and the whole exposed to the hght. Where it 

 reaches in direct rays, it will be found that' little or no growth has 

 occurred; where, on the other hand, the culture has been in the 

 dark, abundant growth occurs. In diffuse light the growth is 

 merely somewhat inhibited. 



A number of experiments in this direction were made at Lawrence, 

 Massachusetts,* with cultures of typhoid and B. coli. 



In two experiments, each with typhoid bacillus and B. coli, water 

 dilutions were made from fresh cultures of the germs, 1 c.c. of this 

 water being placed in Petri dishes in the sun for definite periods. 

 After exposure, the water in the plates was mixed with agar, and all 

 plates were incubated twenty-four hours at 38°, after which the 

 number of colonies was counted. In one experiment the water 

 dilution of typhoid was mixed with melted agar, and plates made as 



* Thirty -fourth Ann. Rep. State Bd. of Health of Massachusetts, 1903, 

 p. 275. 



