24 MICRO-ORGANISMS AND FERMENTATION. 



say at 60 to 70 C., MIQUEL adds 2J to 3 per cent, of carraghen 

 to the nutritive liquid. Slices of potatoes are also used as solid 

 nutritive media. For plate cultures of acid-forming bacteria 

 some litmus or, preferably, carbonate of lime (fine, purified 

 chalk) is added, by which means the colonies of these bacteria 

 stand out clearly from those of other species, in that they 

 appear to be surrounded by a clear, transparent zone. For 

 certain bacteria silicic acid jelly is used as a medium. 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 liquids exclusively as substrata in his 

 work on the organisms of fermentation. In more recent times, 

 solid substrata have been 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 absolutely pure 

 culture to "be introduced, into the flask ? We have, on purely 

 historical grounds, first sketched the conditions for the preser- 

 vation 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 to attain 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, either with a view 

 to observe the individual, the isolated cell through its successive 



