6 MIOEO-ORGANISMS IN WATER 



through a plug of sterile cotton-wool; this principle 

 is extensively employed in the preservation of culture 

 media in flasks, test-tubes, &c., which are protected 

 from aerial microbes by their mouths being closed with 

 sterilised cotton-wool stoppers. 



(4) Sterilisation hy chemical agents. — As in all opera- 

 tions in which chemical substances are used for sterilisa- 

 tion purposes the risk is incurred of traces of such 

 disinfectants escaping removal, and so destroying the 

 organisms under investigation, along with those foreign- 

 forms which it was desired to eliminate, extreme 

 caution must be exercised in the employment of such 

 substances. It is, in fact, only advisable to resort to- 

 them under exceptional circumstances, and in all 

 ordinary operations to depend upon the conscientious, 

 fulfilment of all the minutise required in sterilisation by 

 the usual methods described. In the case of experi- 

 ments on animals, on the other hand, a solution of 

 corrosive sublimate must be used for locally washing the 

 body before making an incision, either for the purposes- 

 of inoculation or dissection in the autopsy ; but in the 

 usual routine of bacteriological investigations the use 

 of this and other disinfectants is not only unnecessary^ 

 but attended with the very greatest risk for the reasons. 

 specified above. 



CULTURE MEDIA 



For the cultivation of all micro-organisms more or 

 less moist materials are necessary, both liquids and 

 solids being employed for the purpose. It might be 

 supposed that it would be easy to find a medium which 

 would suit the requirements of all micro-organisms,, 

 since from some points of view they are all. so similar; 

 but, as a matter of fact, there is the greatest diversity 

 in their tastes, and media which are suitable for the 



