INFLUENCE OF LIGHT 61 



cultures viz., of Tyrothrix scaber in the study of this question. He proved, 

 at the same time, that the duration of exposure to sunlight necessary to kill the 

 microbe is dependent on the composition of the nutrient medium employed for 

 the culture, cells cultivated in bouillon proving less capable of resistance than 

 those of the same species grown in milk. 



Several other explanations have been given regarding the particular and 

 more intimate reactions that occur in a bacterial culture exposed to the rays of 

 the sun. Some observers adhered to the opinions expressed by Downes, and 

 attempted to show that, by exposui-e to sunshine, decomposition products are 

 formed in the medium and act fatally on the bacterial cell. Support for this 

 view is found in the observation made by G. Roux (I.), that the destruction of 

 the germ goes on much more rapidly when there is a concurrent admittance of 

 air ; and an indication pointing in the same direction is afforded by the fact, 

 determined by RICHARDSON (I.), that hydrogen peroxide a substance highly 

 poisonous to bacteria is formed when sterilised urine is exposed to sunlight. 

 On the contrary, other observers WARD (I.) in particular have shown that 

 the presence of such oxidising agents is not essential, but rather that sunshine 

 alone suffices to destroy the vitality of even the strongest bacterial spores. 

 Probably in nature both agencies co-operate in producing the same results. 



The last-named investigator also examined more closely the degree of influence 

 exerted by the individual colours of the spectrum, and found that, in the case of 

 red to green, this action is almost nil, increasing thence to its maximum at the 

 violet end of the blue, and then falling away again in the violet and ultra-violet 

 rays. According to the researches of SANTOUINI and GEISLER (I.), a similar, 

 though less powerful, injurious action is exerted by the electric light; and 

 F. MINCK (I.) has performed several experiments on the effect of the Rb'ntgen 

 rays on bacteria. 



The anti-bacterial influence of sunlight is of the highest importance, especially 

 in regard to the self -purification of rivers. As is well known, the amount of 

 organic matter and the number of bacteria in river-water diminish in proportion 

 as the water increases its distance from the point of contamination. This 

 property, on account of its hygienic and technical importance, has already 

 formed the subject of investigation. HANS BUCHNER (IV. and Y.) in 1892 

 pointed out that all previous explanations of this occurrence had omitted one 

 factor, viz., the influence of light. He showed that a natural water to which 

 about 100,000 cells of Bacterium coli commune an organism constantly and 

 abundantly present in faeces had been added per i c.c., contained, after one hour's 

 sunlight, no living germs. To bring this action into specially prominent notice, 

 he poured peptonised meat-juice-agar-agar, inoculated with a copious supply of 

 typhus bacilli, into Petri basins, on the under side of which were affixed the 

 letters TYPHUS cut out of black paper. The basins were then exposed to the 

 sun's rays for one to one and a half hours, or to diffused daylight for five hours, 

 and afterwards left in a dark room for twenty-four hours. On the paper letters 

 being then removed, their form was found to be marked out bjr the thickly 

 clustered whitish colonies composed of the bacteria that had been protected from 

 the fatal effects of sunlight by the paper cover, and had consequently remained 

 alive, whilst the residual uncovered portion of the medium was destitute of any 

 such colonies. Fig. 29 is a reproduction of the photograph taken by Buchner 

 from one of the plates. The same result was obtained by illuminating the care 

 fully-closed culture basin under water. Experiments made in the clear waters 

 of Lake Starnberg showed that the anti-bacterial influence of the sun's rays 

 extends to a depth of some two metres (about eighty inches) below the surface 

 of the water. Therefore, to the already known factors in the self -purification of 

 rivers viz. sedimentation, oxidising influence of air, consumption of filth by 



