INFLUENCE OF TEMPERATURE. 155 



therefore all life processes are checked. If freezing is prevented by 

 adding salts or other soluble substances which lower the freezing-point, 

 growth may continue even below o. Milk freezes at about 0.5. Bac- 

 teria are found to multiply in it as long as it is not entirely solid. A 

 certain yeast multiplied slowly in salted butter kept at about 6. 



The number of microorganisms that developed at the freezing-point 

 was found to be 



in i c.c. of market milk, up to 1,000 germs. 

 in i c.c. of sewage, up to 2,000 germs, 



in i g. of garden soil, up to 14,000 germs. 



THE MAXIMUM TEMPERATURE is usually about 10 to 15 higher than 

 the optimum. The development of microorganisms above the optimum 

 temperature is not quite normal; there is a great tendency toward invo- 

 lution forms. The mycelium of molds grown near the maximum tem- 

 perature appears unhealthy and pathogenic bacteria lose part of their 

 virulence. This loss of virulence is made use of in the preparation of 

 attenuated cultures for vaccines. 



The maximum temperature varies with different species of bacteria. 

 Most bacteria do not grow above 45, but with some the maximum 

 temperature is considerably lower. Bact. phosphoreum dies if exposed 

 for a few hours at 30; others may require still lower temperatures. The 

 average organisms found in water, soil, milk, and the body, which have 

 their optimum near 30 to 38, do not grow higher than about 45. There 

 are very noticeable exceptions to these, such as the physiological group 

 known as thermophile bacteria. 



These extraordinary organisms have their maximum between 70 and 

 80, a temperature which coagulates albumin. Corresponding to the 

 high maximum the thermophiles have a very high optimum, and the 

 minimum lies with most of these species above 30. These organisms are 

 found in soil, sewage, ensilage and occasionally in milk. They find the 

 temperature suitable for their life only under extraordinary circum- 

 stances, as in fermenting manure piles, in silos, in self-heating hay and 

 similar organic material that develops a high temperature by fermenta- 

 tion. Some hot springs have a very remarkable flora of thermophile 

 bacteria. 



BIOLOGICAL SIGNIFICANCE OF THE CARDINAL POINTS OF TEMPERA- 

 TURE. The importance of the temperature requirements of certain organ- 



