7 8 THE BACTERIA UNDER PHYSICAL AGENCIES. 



earliest investigations therein, published in 1877 and 1878 by 

 DOWNES and BLUNT (I. and II.), who found that the growth of 

 the bacteria is restricted by the influence of diffuse white daylight 

 and is completely stopped by sunshine. The blue and violet rays 

 proved the most injurious, the red and orange rays being weaker 

 in their action. The authors explained the injurious effect of 

 light as an indirect one, in that it strengthens the decomposing 

 power of oxygen, the result being the decomposition and destruc- 

 tion of the bacterial plasma. JAMIESON (I.), in 1882, gave another 

 explanation of the phenomenon by attributing the injury observed 

 to the increase of temperature effected in the cells by the sun's 

 rays. The fallacy of this hypothesis which had been rejected 

 by DOWNES (I.) was demonstrated in 1885 by DUCLAUX (IV.), 

 who was also the first to employ pure cultures viz., of TyrotJtrix 

 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 exposure 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 point- 

 ing 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 par- 

 ticular 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 SANTORINI and 

 GEISLER (I.), a similar though less powerful injurious action is 

 exerted by the electric light; and F. MINOR (I.) has performed 

 several experiments on the effect of the Rontgen rays on bacteria. 



The anti-bacterial influence of sunlight is of the highest im- 

 portance, especially in regard to the self-purification of rivers. 



