MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 9 



mentation, that efforts, based upon the right understanding of 

 the paramount importance 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 

 return to this point in Chap. VI. 



In laboratory experiments, where the object is 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, first 

 in small quantities, then in masses large enough to be trans- 

 ferred at a given time from the laboratory to the brewery. 

 Conditions which are wanting in practice are realised in the 

 laboratory, which is specially arranged for such investigations. 

 We shall now briefly mention these requirements and the 

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

 we shall begin with the final operation and describe the 

 vessels and liquids which receive the previously prepared 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 operator. 



3. STERILISATION. 



The principles of the technology of sterilisation as well as 

 the models of the various apparatus appertaining thereto 

 were described in the old memoirs on spontaneous generation. 



As early as the year 1765, SPALLANZANI argued against 

 the doctrine of NEEDHAM and BUFFON, that living beings came 

 into existence through spontaneous generation in putrefying 



