122 MICRO-ORGANISMS AND FERMENTATION. 



HANSEN'S INVESTIGATIONS. 



When Hansen published, in the year 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 he emphasised the 

 fact that it was not possible to proceed further along the path 

 which they had pursued, but- that the investigations, and 

 especially those commenced by Pasteur and Reess, must, if 

 they were to be carried further, be attacked from a totally 

 different point of view. It was only in the latter end of the 

 year 1881 that he succeeded in finding the key to the solution 

 of the problem. The problem was, in the first place, to 

 devise a method by means of which one could obtain growths, 

 each of which was derived from a single cell, in order to 

 determine by experiment whether these guaranteed 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 in 

 different directions the conditions of life of these organisms. 



1. PREPARATION OF THE PURE CULTURE. 



In the first chapter of this book it was pointed out 

 that the idea had been expressed on various sides, that 

 the only condition for an exact knowledge of the 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 in working with a pure growth 

 obtained from this cell. The different methods which had 

 been employed were also briefly described. 



Hansen has repeatedly pointed out in his papers that the 

 only method ivhich is certain in all cases is to start from 

 the individual cell and to secure the beginning from this. 

 He has devised two different methods for this purpose. In 



