CHAPTER VII. 



STERILISATION. 



An object is said to be sterilised when it has been freed from living 

 germs. It must be remembered that, as a rule, the air is charged with 

 spores of bacteria and fungi, and with the dried but living bacteria 

 themselves, and these being subject to gravity fall to the ground ; 

 therefore every article is covered with them except in those places 

 where bacteria are non-existent, as at the summit of Mont Blanc, or 

 in the Arctic and Antarctic regions. 



As these organisms will germinate rapidly, if provided with the 

 necessary moisture and food material, it is necessary to free from them 

 any object which we intend bringing near or into contact with a nutrient 

 medium ; for they will drop in and soon make the medium useless for 

 the cultivation of other bacteria. In cases of infectious diseases it is 

 manifestly important to remove the bacteria causing these diseases 

 away from our persons, so that our clothes, all materials that come 

 near our bodies, the surrounding atmosphere and walls must be 

 treated in such a way that the risk of infection is diminished. In 

 freeing an object from bacteria, care must be taken that it is not 

 destroyed altogether for the purpose we have in view. Thus, objects 

 like gelatine and cotton wool cannot be sterilised by dry heat, for 

 they would soon get charred and become quite useless; hence, it is 

 important to know the best method of sterilising any particular 

 substance that we wish to free from germs. 



Sterilisation of Air. To procure sterilisation, it is essential that the 

 disinfectant be brought into immediate contact with the bacteria. It 

 is a very common error to imagine that, if a dish of carbolic acid be 

 placed in a room, the room is thereby freed from germs. There is no 

 doubt that all the bacteria which actually fall into the dish will be 

 rendered harmless ; but this number is extremely small compared with 



