STERILISATION AND PURE CULTURE I47 



An alternative method of sterilising a liquid which 

 avoids the use of heat, and has also the advantage of 

 excluding the bodies of bacteria, is by filtering through 

 unglazed earthenware. The liquid to be sterilised is 

 forced under pressure through the filter, whose pores 

 must be so small that they will not allow the smallest 

 bacterium to pass. To be efficient such a filter must 

 be made of the finest " china clay." But in practice 

 sterilisation by filtration is uncertain, because of in- 

 equalities in the size of the pores and imperfections in 

 the filter. Such filters are often attached to domestic 

 water taps to make sure that drinking water contains 

 no harmful bacteria. 



Pure Cultures. — In the early days of the study of 

 bacteria it was impossible for the observer to be sure 

 that he was not dealing with a mixture of different 

 kinds. The cells in a living culture of bacteria might 

 show various forms, or bring about various effects 

 on the culture medium from time to time. This might 

 be due to the powers of a single kind of bacterium, 

 or on the other hand to the gradual disappearance of 

 one species and the multiplication of another (which 

 was also present) as the chemical constitution of the 

 medium was altered owing to the effect of the bacteria 

 upon it. No increase of accurate knowledge of the 

 different kinds of bacteria was possible under these 

 conditions. 



The discovery of the possibility of complete sterilisa- 

 tion of a medium by Pasteur not only proved that all 

 changes of the nature of putrefaction and fermentation 

 were brought about by living organisms, but it enabled 

 perfectly pure cultures of a single kind of bacterium to 

 be made and kept. The making of pure cultures for 

 the study of bacteria was introduced about the year 



