RELATION TO ENVIRONMENT CLASSIFICATION 39 



ganisms. Thus, according to Smith 29 and others, ten and twenty 

 minutes' exposure to a temperature of 70 C. is necessary to destroy 

 tubercle bacilli in a fluid medium. For the effectual destruction of 

 spores by moist heat, a temperature of 100 C., or boiling point, is 

 usually necessary. 



Low temperatures are much less destructive than the high ones, 

 and are even in a number of cases useful in keeping bacteria alive for 

 long periods, inasmuch as metabolic processes are inhibited and life 

 is maintained without actual development in a sort of resting state. 

 Actual destruction by low temperatures rarely takes place. The 

 exposure of diphtheria, typhoid, and other bacilli to temperatures as 

 low as 200 C. below zero has been carried out without destruction 

 of the microorganisms, a fact which is of great importance in consid- 

 ering the possibility of infection by the vehicle of ice. Meningococci 

 and gonococci, on the other hand, die out rapidly when exposed 

 to C. 



Relation to Pressure. High pressure does not injure bacteria. 

 Certes 30 found that a pressure of two atmospheres had no influence 

 upon the growth of anthrax bacilli suspended in blood. 



Relation to Moisture. For the growth and development of all 

 bacteria, the presence of water is necessary. Nutritive materials can 

 not be absorbed by an osmotic process unless in a state of solution. 

 While complete dryness does not permit growth, its destructive 

 action upon various bacteria is subject to great differences. The 

 effect of complete drying upon bacteria will be found more fully dis- 

 cussed on page 76. In the same place may be found a discussion of 

 the effects of light, electricity, x-ray, and radium rays upon bacteria. 



THE CLASSIFICATION OF BACTERIA 



Too simple in structure, too varied in biological properties to be 

 definitely identified with either the vegetable or animal kingdom, the 

 bacteria are placed at the bottom of the scale of all living beings. 

 Closely linked on the one hand to the plant kingdom by the yeasts 

 and the molds, and on the other to the animal kingdom by the pro- 

 tozoa, they themselves combine, within one arid the same division, 

 attributes so widely divergent as to structure, metabolism, and bio- 

 logical activity that their grouping is more a matter of working con- 



29 Th. Smith, Jour, of Experimental Med., No. 3, 1899. 

 80 Certes, Compt. rend, de 1'acad. d. sc., 99, Paris, 1884. 



