ESSAYS ON BACTERIOLOGY. 21 



ences to whipli they are subjected. Two groups of 

 factors have, therefore, been studied in the effort to 

 obtain more definite knowledge of the durability of 

 infectious material, that is, the tenacity of life of 

 micro-organisms. Elaborate investigations have been 

 made of the relation between various bacteria and the 

 media containing them, and thus has opened up a 

 large field for the study of the growth and durability 

 of bacteria in various food stibstances, in milk and in 

 water. Thus, in milk many find a congenial home; 

 in water some retain their vitality for a long time, 

 others soon disappear. But aside from culture soils, 

 certain conditions have much to do with the life-his- 

 tory of bacteria, the most important being tempera- 

 ture, moisture, and the presence or absence of air. 

 Some grow only at high temperatures, that is, at about 

 body-heat; others at a much lower point. Some re- 

 tain life in the absence of any notable amount of mois- 

 ture, others quickly die when dried. Some grow only 

 in the presence of air, others flourish without it, and 

 hence the division into aerobic and anaerobic bacteria. 

 Then, too, considerable study has been made of the 

 destiny of bacteria in the body after death. The re- 

 sults are not as yet very conclusive, but the weight of 

 evidence seems to be that many kinds of germs soon 

 disappear. Esmarch has recently gone over this 

 ground again, his conclusion being that just stated ; an 

 inference being that graveyards are not as dangerous 

 as has been supposed. 



