LIGHT 



101 



able that the crystals formed in the freezing of water play a great 

 part in the mechanical injuring of the bacteria. The freezing of the 

 soil increases not only the number of bacteria within it, but the 

 ammonifying and nitrogen-fixing powers of the soil. Whether or 

 not this will vary with the water content of the soil has not yet been 

 answered, but it is likely that as the moisture content increased 

 the greater would be the injurious influence of the low temperatures. 

 Light. That light greatly affects the metabolism of the living 

 cell is well known. However, bacteria are even more sensitive to 

 light than are most cells. Diffused daylight exerts a hindering effect 



FIG. 15. Thickly sown plate culture of typhus bacilli on agar-agar. Covered 

 with paper letters and exposed to the sun's rays for one and a half hours, then kept 

 twenty-four hours in the dark, whereupon development of thickly congregated 

 whitish colonies was found only at the parts covered by letters. (After H. Biichner.) 



upon bacterial growth and metabolism, whereas direct sunlight is 

 highly injurious to certain bacteria, many microorganisms being 

 killed almost instantly when exposed to the full action of the sun's 

 rays. The different colors of the spectrum do not act alike. The 

 longer rays, from red to green, are practically without influence 

 upon bacteria, but the blue and violet rays have the most marked 

 germicidal power. 



Since light has no effect upon bacteria in a vacuum, it has been 

 inferred that the changes brought about in the bacterial cell are 

 primarily oxidation changes which are incompatible with the life 

 of the cell. This reaction is brought about more rapidly in those 



