296 BACTERIOLOGY 



the dark. The first definite researches on the subject, 

 however, were made in this country by Downes and Blunt 

 in 1877, since when many investigations have been carried 

 on, with the result that the power of light to inhibit the 

 growth of many micro-organisms has now been placed 

 beyond a doubt. 



Thus Koch l found that tubercle bacilli are killed on 

 exposure to direct sunlight, varying from some minutes 

 to a few hours, according to the thickness of the layer, 

 while diffuse daylight required from 5 to 7 days to produce 

 the same effect. Similar results have been obtained with 

 the typhoid bacillus by Janowski, 2 who exposed to diffuse 

 daylight three culture tubes, one uncovered, one wrapped 

 in white, and one in black paper, and also a culture in a 

 U tube one limb of which was covered, and found that the 

 uncovered culture or portion of a culture was greatly re- 

 tarded in its development, while 4 to 7 hours of direct 

 sunlight killed it altogether. Buchner 2 exposed a plate 

 culture of the same bacillus in a Petri's capsule, to which 

 a cross of black paper had been attached, and then kept it 

 for 24 hours in the dark, when it was found that after 1 to 

 1^ hour of sunlight, or 5 of diffuse daylight, no colonies 

 developed except on the part of the plate protected by the 

 paper cross, all the rest having been destroyed. He also 

 obtained confirmatory results with Bacillus coli commune, 

 Bacillus pyocyaueus, and Vibrio cholera Asiatics, finding 

 that some water which contained 100,000 elements of the 

 first-named per c.cm. yielded no growth after an hour in 



1 Vcrhandlung. d. 10. medic. Congress. Berlin, 1890, Bd. I. p. 42 (quoted 

 by Giinther). 



2 For bibliography up to date, see P. Frankland and H. M. Ward, 

 * Report on the Bacteriology of Water,' Roy. Soc. Proc., vol. li. No. 310, 

 pp. 199 n. and 237-9 ; also the other papers by the latter (to be cited pre- 

 sently). It may be mentioned, however, that Janowski's paper appeared in 

 Centralb. f. Bakt. u. Parasitenk., Nos. 6-8, 1890 ; and Buchner's ibid. 

 Bd. xii. 1892, and Bd. xi. No. 25. 



