ESSAYS ON BACTERIOLOGY. 113 



especially their growtli and multiplication, is chiefly 

 under the control of four factors, namely, food, tem- 

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

 And again like most other living beings, the evidence 

 of their activity is not chiefly mechanical, but is found 

 in the field of biological chemistry. 



Many bacteria grow at the ordinary temperature 

 of the house or the surrounding air, while some re- 

 quire for their multiplication temperatures approxi- 

 mating that of the bodies of warm-blooded animals. 

 They do not multiply in the dried state, being in this 

 respect again like other seeds. Some require while 

 some reject a free access of air. Some welcome while 

 some quickly die under direct sunlight. Indeed, the 

 more we study them the more clearly do we see that 

 they are under the laws which prevail throughout the 

 organic world. 



The attempt to classify the bacteria on a strictly 

 scientific, that is, a botanical basis, has thus far been 

 unsatisfactory. They are therefore classified accord- 

 ing to certain of their more conspicuous characteris- 

 tics. Of these groupings the most common, and the 

 one generally used, is that based upon their form. 

 According to this system we describe most bacteria as 

 belonging to one or the other of three great groups. 

 These are : the round organisms or micrococci, J;he rod- 

 shaped organisms or bacilli, and those having a spiral 

 or corkscrew shape, called spirilla. Thus we speak of 

 the micrococci of erysipelas, the bacilli of diphtheria. 



