PHYSIOLOGY OF MICROORGANISMS 55 



light is used for the destruction of bacteria in water. The 

 source of light is the mercury-vapor lamp, which is espe- 

 cially high in violet and ultra-violet rays, which are most 

 efficient in the destruction of bacteria. Glass prevents the 



Fig. 18. Effect of Light 



The agar plate was seeded over the entire surface with bacteria. The center 

 of the plate was protected and the plate was exposed to the direct rays of the 

 sun for one hour. All organisms except those protected from the sun have 



been killed. 



passage of these rays to such an extent as to make a lamp 

 with a glass tube of little value, while one with a quartz 

 tube will be very effective. 



The germicidal action of sunlight in the destruction of 

 disease-producing organisms in houses and stables has been 

 greatly over-emphasized. It requires but a thin layer of 

 dust to destroy the effect of the light. Again, but a small 



