MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 9 



as to keep continually in a state of absolute purity. It is, 

 however, one of the most salient features in the present 

 development of the industry of fermentation, that efforts 

 based upon the right understanding of the paramount im- 

 portance of the fermentation organisms are being made to 

 emancipate the chief useful species from the action of injurious 

 forms. The very great importance of this was not, however, 

 appreciated until Hansen, through methodical selection of 

 certain types of yeast, showed that such a pure growth insures 

 far greater certainty and uniformity than the impure and 

 unknown yeast mixtures hitherto used. We shall come back 

 to this point later on (Chapter VI.). 



In experiments in the laboratory, where the object is 

 likewise to prepare cultures of fermentation organisms, 

 greater demands may naturally be made than on a practical 

 scale. In this case it is necessary to work with absolutely 

 pure cultures, partly in small quantities, partly in masses 

 so large that they may be transferred at a given point of 

 time from the laboratory to the brewery. Conditions which 

 are wanting in practice are sought to be realised in the 

 laboratory, which is specially arranged for such investigations. 

 We will now briefly mention these requirements and the way 

 in which they are met, and, for purely historical reasons, 

 we will begin with the last link, viz., with the vessels and 

 liquids which receive the originally-prepared small pure 

 cultures, and the expedients to be employed in their cultiva- 

 tion. It is necessary that these vessels and liquids be sterile 

 before the inoculating substance is introduced, i.e., they must 

 be freed from all living germs ; also that the various utensils 

 and the air in the place where the work is performed should 

 contain as few living germs as possible. The same applies 

 of course to the clothing and hands of the experimenter. 



3. STERILISATION. 



The principles of the technology of sterilisation as well 

 as the models of the various apparatus appertaining thereto 



