72 INFLUENCE OF PHYSICAL AGENTS 



the direct rays. We are certainly not justified in supposing that the 

 sunshine causes disinfection on such a scale as would lead to the self- 

 purification of rivers (40). 



In cultures, both spores and vegetative bacterial cells are killed by 

 exposure to direct sunlight for two or three hours, and it is to the actinic 

 rays, not to the heat, that the deleterious effect is due. All parts of the 

 spectrum, however, are not equally efficacious, for if the light before falling 

 upon the bacteria be allowed to pass through coloured solutions, such 

 as bichromate of potash or ammonium cupric oxide, we find that it is 

 only the more refrangible rays that have the power of arresting growth. 

 It has been shown that freshly inoculated broth cultures of typhoid bacilli, 

 exposed for eight hours to light that had passed through a solution of 

 bichromate of potash, grew well, the broth becoming turbid with the 

 organisms. If instead of bichromate an ammoniacal copper solution was 

 used, the bacilli would not grow at all, the broth remaining clear even after 

 five days. Leaving aside the question as to whether the culture medium 

 itself is injuriously changed, it is evidently the blue end of the spectrum 

 the photochemically active rays that destroys the bacteria. These 

 rays too inhibit the formation of spores in the common mould-fungus 

 (Botrytis cinerea) during the day. Only at night is it able to develop the 

 reproductive cells. In some fungi, on the other hand, light is necessary for 

 fructification. Such are the little (1-2 mm. high) Pilobolus that projects its 

 ripe sporangia more than a metre into the air, and Coprinus, both forms 

 common on horse-dung. These become etiolated in the dark just as would 

 a chlorophyll-bearing plant. Such examples show that it is not possible 

 to formulate definite laws as to the action of light on colourless fungi (41). 

 Possibly there are other 'photophile' bacteria besides the pigmented 

 Erythrobacteria and their highly sensitive ally, the B. photometricum of 

 Engelmann. As a general rule it is necessary to keep bacterial cultures in 

 the dark, or protect them at least from intense light Weak diffused light 

 does no harm. 



For practical purposes of disinfection on a large scale, light (even 

 bright sunshine) is unsuitable. 



Strong electric currents (44) are fatal to bacteria, their protoplasm being 

 killed and no doubt altered in the same way as that of other plant cells is; 

 although some of the deleterious effect may be due to the electrolytic 

 dissociation of the nutrient media, and to the increase of temperature that 

 the current may cause. These drawbacks are a great hindrance to the 

 employment of electric currents for the disinfection of food-stuffs. 

 Attempts have been made (43) in distilleries to destroy by this means 

 the harmful bacteria without injuring the yeast cells. Currents of about 

 5 amperes were employed, but the practical difficulties of the process are 

 very great, and have not yet been entirely overcome. 



