142 MICRO-ORGANISMS AND FERMENTATION. 



only right that they should form the ground-work of the 

 following section of this book. 



When HANSEN published, in 1878, his treatise on "Micro- 

 Organisms in Beer and Wort," he pointed out the uncertainty 

 which prevailed in the works of earlier writers, concerning the 

 true Saccharomycetes ; and asserted that it was impossible to 

 proceed further along the path which they had pursued, but 

 that the investigations (especially those commenced by PASTEUR 

 and EEESS) must be attacked from a totally different point of 

 view if they were to lead to any definite issue. It was only 

 in the latter end of the year 1881 that he succeeded in 

 finding the key to the solution of the problem. In the first 

 place the problem was to devise means by which growths 

 could be obtained, each of which was derived from a 

 single cell, in order to determine by experiment whether these 

 incontestable/ pure cultures exhibited constant characters that is 

 to say, how far the Saccharomycetes occur as species, varieties, 

 or races and, should this prove to be the case, to determine 

 what these characters are. When this problem was solved, the 

 next was to devise a method for the analysis of yeast and to 

 study the conditions of life of these organisms. 



1. PREPARATION OF THE PURE CULTURE. 



Eeference was made in our first chapter to the prevalence 

 of the idea that the one condition for an exact knowledge of 

 micro-organisms, hundreds or thousands of which we find in 

 every drop when examined under the microscope, consists in 

 the isolation of a single cell, and subsequent study of a pure 

 growth obtained from this cell. The different methods which 

 had been employed were also briefly described. 



HANSEN has repeatedly pointed out that the only inevitably 

 sure method is to start from the individual cell and to secure the 

 beginning from this. He has devised two different methods 

 for this purpose. In his first method a liquid medium was 

 employed, and in his second method a solid medium, for the 

 cultivation ; in both cases the culture had been diluted as 

 previously described (Chapter I., " Preparation of the Pure 

 Culture "). 



