774 BACTERIA 



skin, in all our food-stuflfs, in the alimentary canal of animals, and 

 all over the globe wherever dust can find a lodging-place. They 

 are, however, absent from the tissues of healthy animals and 

 plants ; moreover, in the air on high mountains, and in water 

 from springs and deep wells which has been filtered through 

 great depths of soil and rock, their numbers are few. 



6. For the development and multiplication of bacteria certain 

 external conditions are needful, the chief of which are given 

 below. 



(a) Darkness. — While a few bacteria appear to need a certain 

 intensity of light to enable them to carry on their functions, 

 by far the larger number of species require darkness for their 

 proper development, being checked in their growth even by 

 diffuse daylight and entirely destroyed by exposure for a few 

 hours or days to direct sunlight. 



ip) Water. — For the continuance of active life in all kinds 

 of organisms water is necessary, but the spores of bacteria are 

 able to withstand dessication for a long time, in some cases for 

 several years. In the vegetative state, however, dryness checks 

 the vitality of these organisms, and if continued for a few weeks 

 results in their death. 



if) Heat. — The temperature most suited to growth varies 

 with each species of bacterium. Some met with in the soil 

 are able to multiply at freezing-point, while the tubercle bacillus 

 does not grow below a temperature of 30° C. and thrives best 

 when kept about 37° C. One species. Bacillus thermophilus, 

 which has been obtained from soil, sewage and other sources, 

 develops extensively at 70° C, a temperature which is sufficient 

 to coagulate egg-albumin and cause the death of all animal 

 cells. 



The temperatures most favourable for the growth of a number 

 of common species lie between 25° atid 35° C. As the freezing- 

 point is approached division ceases but the organisms are 

 capable of enduring very low temperatures without losing their 



