32 GENERAL MORPHOLOGY AND BIOLOGY. 



generations under observation within twenty-four hours. If 

 we accept De Bary's definition of a species, we can have little 

 difficulty in saying that such exist among the bacteria. The 

 definition is : " By the term species we mean the sum total 

 of the separate individuals and generations which, during 

 the term afforded for observations, exhibit the same peri- 

 odically repeated course of development within certain 

 empirically determined limits of variation." 



The Death of Bacteria. The death of bacteria is 

 usually judged of by the fact that, when they are trans- 

 ferred to a fresh quantity of an artificial medium in which 

 they previously grew, no growth takes place. Under the 

 microscope the counterpart of this of course would be the 

 cessation of division when surrounded by such a medium. 

 All bacteria can be killed by heat, drying, starvation, and 

 chemical agents, as we have seen. Great attention has been 

 paid to the latter, which are usually antiseptics, though 

 germicides would be a more proper term to apply. The 

 action of such agents depends on the variety of bacterium 

 to be killed, on its state of nutrition, whether it is in a 

 vegetative or a spored condition, on the temperature at 

 which the agent acts, on the medium in which it acts, and 

 on the nature of the chemical agent itself. Among inorganic 

 bodies the salts of the metals with high atomic weights act 

 more potently than those with lower, and the most powerful 

 antiseptic bodies are probably the perchloride and periodide 

 of mercury. The reaction of the agent is a point of great 

 importance; as a general rule, the more powerful an acid is, 

 the greater is its capacity as a germicide. The importance 

 of oxidising and reducing agents as germicides has probably 

 been overestimated. Among organic bodies the members 

 of the aromatic series are all more or less potent the 

 favourite member for practical use being carbolic acid. In 

 comparing the action of antiseptic agents the all-important 

 point is their relative molecular constitution. From the 

 number of conditions we have enumerated, which must 

 be considered in estimating antisepticity, it is evidently 

 impossible to make definite statements as to the value of 



