66 BACTERIA AND SOIL FERTILITY 



crobes are destroyed more easily in an acid or alkaline solution 

 than in a neutral one. Hence, in determining the thermal 

 death-point of bacteria a neutral salt-solution is used. 



Potatoes may be dropped into boiling water and left for one 

 hour and still be infected with bacteria. They had on their sur- 

 face resistant spore-bearing bacteria. We find, therefore, that 

 the temperature necessary to kill also varies with the organism 

 and the condition of the organism when heated. 



When one is desirous of freeing any substance from these heat- 

 resistant organisms one of two methods is used: (i) Heating in a 

 steam autoclave at an increased pressure for a given length of 

 time, depending upon the substance to be sterilized. This permits 

 the heating of the medium to a temperature above ioo° C. (2) 

 The intermittent method of sterilization may be used. This is 

 carried on by placing the substance to be sterilized in live steam 

 for a definite time on one day and then setting aside in a warm 

 place for twenty-four hours, during which time the spores germi- 

 nate. Then the heating processes repeated. This is usually 

 continued for even a third time, when the medium is usually 

 found free from microorganisms. 



Light.— The direct rays of the sun are injurious to living 

 tissues, as every boy can testify who has spent too many hours in 

 swimming with the sun beating upon his back. Moreover, we 

 often find individuals who spend much of their time indoors 

 and when they go on a fishing trip they often return with a 

 blistered nose. Now bacteria are even more sensitive to light 

 than are the cells of the human body. Diffused sunlight hinders 

 their growth, and the direct sunlight is highly injurious. Some 

 bacteria are killed almost instantly when exposed to the full action 

 of the sun's rays. Hence, we find the foundation for the oft- 

 quoted Italian proverb, "Where the sun does not enter, the doctor 

 does.'' Sunlight probably plays a major role in the sterilizing of 

 water and the destroying of pathogenes which find their way into 

 the air during coughing, speaking, and sneezing. All light rays 

 are not of equal importance, the blue to violet rays being more 

 active than the yellow to red. Therefore, the injurious effect 



