STERILISATION. 13 



culture is transferred with every precaution from the moist 

 chamber to a flask containing sterile liquid (see detailed 

 description in a later section). 



3. Sterilisation. 



The principles of the whole technology of sterilisation, 

 as well as the different kinds of apparatus required, were 

 described in the early memoirs on spontaneous generation. 



The details of the development of this subject in its his- 

 torical setting are given with the description of the theories 

 of fermentation in Chap. v. 



Sterilisation of objects, whether a liquid or a piece of 

 apparatus, means the riddance therefrom of all germs capable 

 of development. This may be carried out either by removing 

 all germs by mechanical means or by killing them by heat, 

 or by the use of antiseptics. The choice of treatment is 

 determined by the composition of the object to be sterilised, 

 and obviously those means will be chosen that will render the 

 adhering germs harmless, whilst producing the minimum of 

 change in other directions. Sometimes, with this in view, 

 the sterilisation can only be partial ; for instance, if the pro- 

 perties of the liquid are changed to a great extent, by heating 

 to the temperature at which the germs present would be killed, 

 the lowest temperature must be found by experiment at which 

 the organisms are so greatly enfeebled that they are no longer 

 able either to develop or to affect the liquid. This is the 

 object of pasteurisation, which will be discussed later on. 



One circumstance which often presents great difficulties 

 to complete sterilisation is this that the great majority of 

 micro-organisms occur in two different forms of growth, 

 vegetative cells and spores. Whilst the former are usually 

 killed at a temperature below the boiling point, the latter, 

 and especially the spores of bacteria, can withstand prolonged 

 heating at the boiling point ; thus Fliigge has isolated a 

 species of bacteria from milk, the spores of which withstood 

 boiling for four hours. When such spores are encountered, 

 it is necessary either to boil for several hours, or to adopt a 



