22 MICRO-ORGANISMS AND FERMENTATION. 



5 to 10 per cent, of gelatine. Similar liquids, or more 

 frequently, meat-extract with an addition of peptone, are 

 employed for bacteriological investigations ; this mixture is 

 neutralised with sodium carbonate. Either gelatine or agar- 

 agar is used for rendering the medium solid. Solid nutritive 

 substrata are the best for the study of mould-fungi, in most 

 cases preferably sterilised black bread. Where liquids are 

 employed, the most suitable are beer-wort, fruit decoctions, or 

 mixtures of sugar with an addition of tartaric acid or tartrates. 

 Pasteur used exclusively liquids as substrata in his investi- 

 gations on the organisms of fermentation. Later, solid 

 substrata were very extensively employed, and in this respect 

 Koch has given many practical illustrations. 



We have now briefly explained how our micro-organisms 

 are cultivated, and guarded against contamination from the 

 liquid itself, from the vessels and apparatus, from the air, 

 and from the experimenter. We have now before us the 

 first and most important question: How are we to obtain 

 the first absolutely pure culture to be introduced into the 

 flask? I have, on purely historical grounds, first sketched the 

 conditions for the preservation of the pure culture, because 

 these were known long before a certain method for preparing 

 the pure culture itself had been discovered. 



In this respect it will be instructive to see how we have 

 advanced step by step, and we will again take up the subject 

 historically, from the moment when really rational endeavours 

 were made for the attainment of this object. 



7. PREPARATION OF THE PURE CULTURE. 



It is only by starting with one, individual cell that we 

 can be certain of obtaining a really pure culture, and such 

 a culture is the indispensable condition for exact scientific 

 investigations of the micro-organisms. These investigations 

 may, as stated above, be carried out for different purposes, 

 namely, with a view to observe the individual, the isolated 

 cell through its successive phases of development, morpho- 



