BACTERIOLOGY. 59 



these qualities has not yet been discovered. Substances 

 used to kill insects are called insecticides. Nearly all 

 the disinfectants are insecticides; formalin is a notable 

 exception. The most important insecticides are given 

 in a later chapter. 



Towards agents which have a harmful action upon 

 them, i.e., either interfere with their growth (antisep- 

 tics), or destroy them (disinfectants or germicides), 

 bacteria exhibit distinct differences in resistance — a 

 characteristic which varies with the species. Species 

 which produce spores, are not more resistant to injurious 

 agencies when in the vegetative state, than are the 

 ordinary non-sporulating bacteria. But in the form 

 of spores their power to resist destruction is often 

 marvelous. However, it is not only the sporogenous 

 form which is difficult to deal with; there are a few 

 species which, although they do not produce spores, 

 are nevertheless almost as difi&cult to kill. The tubercle 

 bacillus is one of these, and it is due to this fact, in con- 

 junction with the universal prevalence of tuberculosis, 

 that the tubercle bacillus has a wider distribution in 

 nature than any other specific pathogenic micro-or- 

 ganism. 



The most wide-spread destruction of bacteria is 

 brought about through natural forces. Where actual 

 destruction is not accomplished, the same influences so 

 modify their characteristics as to render them inert. 

 It is in this way that pathogenic bacteria are robbed of 



