EFFECT OF PHYSICAL AGENTS 29 



have been exposed to sunlight remaining sterile. Although 

 this action of sunlight may sometimes be due to chemical 

 changes in the medium, resulting in the production of 

 ozone or other germicidal bodies, the experiments of 

 Marshall Ward and others have conclusively shown that 

 germicidal action may be caused by the direct action of 

 the light, the violet and ultra-violet rays being those 

 concerned, and the red end of the spectrum having no 

 effect. Exposure to ultra-violet rays for short periods 

 may produce mutations of the anthrax bacillus (see p. 6). 

 The Rontgen rays seem to have little or no influence 

 upon bacteria, but the results that have been obtained 

 are somewhat contradictory. 



The radium emanations with prolonged exposure and 

 near contact are germicidal to non-sporing organisms. 1 



Electricity, per se, has also usually little effect. When 

 the current is passed directly through the cultures elec- 

 trolysis takes place, and the products formed may destroy 

 the bacteria ; currents of high potential, however, may 

 inhibit growth. 



Living motile bacilli are very sensitive to induced cur- 

 rents of electricity, immediately orientating themselves in 

 the direction of the current, while dead or paralysed bacilli 

 are unaffected. 



Bacterial Products 



The chemical changes produced by micro-organisms 

 are chiefly analytic or destructive the formation of 

 simpler from more complex bodies. This analytic 

 faculty is present to a marked degree in the process 

 known as putrefaction. Putrefaction is a term applied to 

 the decomposition of organic, especially protein, matter 

 after the death of the animal or plant. It is usually 

 accompanied by the evolution of foul-smelling gases and 

 by solution of the solid material. A large number of 



1 See Green, Proc. Roy. Soc. Lond., vol. Ixxiii, 1904, p. 375. 



