INFLUENCE OF LIGHT. 79 



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 BUOHNER (IV. 

 and V.) 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 speci- 

 ally 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 ex- 

 posed 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 by 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 algae, &c. all of which are more concerned with alterations 

 of chemical composition must be added the influence of sunlight 

 in diminishing the number of bacteria. A critical review of the 

 most important labours and researches performed in respect of the 

 self -purification of rivers is given by E. DUCLAUX (V.). 



All the pathogenic Schizomycetes seem to succumb under the 

 influence of sunlight. This has been shown by Arloing and 

 Ward in respect of Bacillus anthracis ; Gaillard for B. typlii 

 abdominally; Pansini for Vibrio cholerce asiaticce and a fungus 

 giving rise to white pus in wounds (Staphylococcus pyogenes 

 albus} ; Chmiliewski for the organism which induces the for- 

 mation of yellow pus (St. pyogenes aureus), and the bacillus of 

 erysipelas (Streptococcus erysipelatis) ; Rob. Koch for Bacillus 

 tuberculosis; Charrin for the organism producing swine-erysipelas ; 

 and others. Most of the non-pathogenic fission fungi also suc- 

 cumb to the influence of light. GALEOTTI (I.) arranged a number 

 of chromogenic species in the following descending series, the 



