102 DISINFECTION 



laws and principles to any one who, whether engaged in medicine, hygiene, dairy- 

 farming, food-preservation, or agriculture, is confronted with the problem of 

 controlling bacterial activity. 



Physical Agencies 



Light. — Downes and Blunt in 1877 (1877, 1878) showed that exposure of a 

 putrescible fluid to sunlight was sufficient to sterilize it. They observed that this 

 effect was produced only in the presence of air, and were therefore led to regard 

 the germicidal property of light as depending on oxidation. Duclaux (1887) in 

 1886 showed that in sunlight vegetative bacilli were killed more rapidly than 

 spores. The following year Roux (1887) exposed anthrax spores in a nutrient 

 medium to the sun. Some were contained in glass tubes with plenty of air 

 above the level of the liquid, others in glass tubes containing no free air ; the 

 former were destroyed in 29 hours, the latter survived longer than 83 hours. 

 This was a confirmation of the work of Downes and Blunt, but Roux went 

 further ; he found that if nutrient broth was exposed to the sun in a layer 5 mm. 

 deep for 3 or 4 hours, it became changed in such a way as no longer to permit 

 of the germination of anthrax spores, though still remaining suitable for the growth 

 of the vegetative bacilli. This antiseptic property was lost after the broth had 

 been allowed to stand for a time in the dark. Broth exposed to the sun in a sealed 

 glass tube containing no free oxygen was unaffected. It was clear, therefore, 

 that not only was sunlight in the presence of air able to destroy anthrax spores, 

 but that it was able to produce an alteration in a nutrient medium — an alteration 

 which was of a transient nature, suggestive of the activity of some volatile or 

 unstable compound. This action of sunlight was reinvestigated by Burnet (1925) ; 

 working with staphylococci, he found that they would not grow on agar plates that 

 had been exposed to the sunlight, though growing quite satisfactorily on control 

 plates that had been kept in the dark. He was able to show that the reason for 

 this is that under the influence of sunlight hydrogen peroxide is produced, and 

 that this substance is so powerful that its inhibitory effect is noticeable even in a 

 dilution of 1-40,000 (see Chapter 3). 



To return to the action of light on the bacteria themselves : Ward in 1892 

 exposed gelatin and agar plates seeded with anthrax spores to the autumn sunlight 

 for 6 hours, each plate being shaded in such a way that only part of it received 

 the direct rays of the sun. After incubation, a growth of anthrax bacilli was 

 found to have occurred in the protected but not in the exposed portion. The 

 inhibitory action of the sun was not due to the heat rays, because the temperature 

 of the plates at no time rose above 18° C. ; nor was it due to desiccation or other 

 alteration of the medium, since exposed plates seeded with fresh spores proved 

 quite suitable for growth. Ward therefore concluded that the germicidal effect 

 of the sun was due to its actinic rays. Further work showed that if a spectrum 

 was thrown across an agar plate, the inhibitory effect of the light was stronger 

 at the blue than at the red end. This was the first demonstration of the selective 

 action of violet light. Some years later Barnard and Morgan (1903), working 

 with the arc spectrum of carbon and of various metals, made more extensive 

 observations, which led them to conclude that the bactericidal action of light was 

 almost entirely due to those radiations in the ultra-violet region which are included 

 between the wave lengths 3287 and 2265 A.U., that is, the light between the visible 



