EFFECT OF PHYSICAL ENVIRONMENT 117 



or conversely 



g= L 



By use of these formulae for determining the value of g in 

 any growing culture of microorganisms we may detect the 

 effect of changes in environment. Within certain limits, 

 increasing the temperature at which the culture is kept will 

 increase the rate of growth, that is, g will diminish, and 

 the smaller g is, the more favorable are the conditions. 

 Conversely, the larger g is, the more unfavorable are the 

 conditions. 



When organisms are placed under sufficiently unfavor- 

 able conditions they cease to multiply and begin to die in- 

 stead. In most of the cases which have been carefully 

 studied the bacteria die off in accordance with a definite 

 law, which may be stated as follows : with a given kind of 

 organism under uniform conditions, the number of bacteria 

 present in a culture will always be reduced by one-half in 

 equal periods of time, that is, no matter how many bacteria 

 there are at the beginning of a definite period of time, one- 

 half that number will always be alive at the end of the 

 proper interval. For example, suppose that two cultures, 

 one containing a million bacteria and the other a thousand 

 bacteria are subjected to* the same unfavorable conditions. 

 It is found that at the end of a definite period of time, say 

 ten minutes, the bacteria in the less concentrated suspension 

 average five hundred. It will be found that in the same 

 perod of time the other culture has also been halved, that is, 

 there are 500,000 bacteria left. Another way of stating is 

 this : during each equal interval of time a definite percent- 

 age of those bacteria living at the beginning of the period 

 will be killed. If we wish to compare unfavorable condi- 

 tions in their effect upon the death of microorganisms we 

 may compare the length of time required to reduce the 



