BACTERIA AND FERMENTATION 1 23 



quite possible that it will be to the exclusion of the desired 

 culture. Hansen has devised a much improved process for 

 securing a pure culture of yeast which depends upon dilu- 

 tion. We believe Lister was one of the first who, in the 

 seventies, introduced some such plan as this. Hansen em- 

 ployed dilution with water in the following manner: 



Yeast is diluted with a certain amount of sterilised water. 

 A drop is carefully examined under the microscope, a single 

 cell of yeast is taken, and a cultivation made upon wort. 

 When it has grown abundantly a quantity of sterilised water 

 is added. From this, again, a single drop is taken and 

 added, to, say 20 cc. of sterilised water in a fresh flask. This 

 flask will contain we will suppose ten cells. It is now vigor- 

 ously shaken, and the contents are divided into twenty por- 

 tions of I cc. each, and added to twenty tubes of sterilised 

 water. It is highly probable that half of those tubes have 

 received one cell each. In the course of a few days it can 

 be seen how far a culture is pure. If only one colony is 

 present, the culture is a pure one, and as this grows we 

 obtain an absolutely pure culture in necessary quantity. 

 Even when the gelatine-plate method is used it is desirable 

 to start with a single cell (Hansen). The advantage of 

 Hansen's yeast method over Koch's bacterial-plate method 

 is that it has a certain definite starting-point. This is 

 obviously impossible when dealing with such microscopic 

 particles as the bacteria proper. 



A third matter in the differentiation of yeast species is 

 the question of films. Hansen set to work, after having 

 obtained pure cultures and ascospores, to examine films ap- 

 pearing on the surface of liquids undergoing fermentation. 

 The object of this was to ascertain whether all yeasts pro- 

 duced the same mycelial growth on the surface of the 

 fermenting fluid. To produce these films the process is as 

 follows : Drop on to the surface of sterilised wort in a flask 

 a very small quantity of a pure culture of yeast ; secure the 



