52 MICRO-ORGANISMS AND FERMENTATION. 



more convenient and more certain process than lifting the 

 cover-glass, for it is possible to transfer the colonies without 

 inverting it. On one cover-glass there may be 50 to 60 well 

 isolated germs. When the colonies are conveyed to the flasks 

 by means of a small piece of platinum or copper wire, which 

 has been previously ignited and cooled, the culture is moment- 

 arily in the air, and is then exposed to contamination. But 

 the danger of contamination at this, the single weak point, 

 is reduced to an insignificant minimum, and disappears if 

 the operation is performed in a small enclosed sterile space ; 

 for instance, in a small cupboard with glass sides sufficiently 

 large to admit the apparatus and the operator's hands. In 

 this way the transference of the colonies is effected with all 

 possible security. From the first flask the culture can be 

 transferred without contamination to a continually increasing 

 number of larger flasks. 



For the pure cultivation of brewery, distillery, and wine 

 yeasts, vigorous cells must be conveyed to the gelatine in the 

 moist chamber. According to J. C. Holm, on an average 

 only about 4 per cent, of the inoculated cells do not develop, 

 whilst from a growth of yeast that is taken at the end of the 

 fermentation, in which the cells are weakened, about 25 per 

 cent, do not develop. It is usually preferable to convey a 

 small average sample of the yeast into wort or must, and 

 then to use the very young growth, which is developed 

 when the first trace of fermentation is observed. To decide 

 whether any of the selected yeasts are of value for industrial 

 purposes, a large number of cells must be isolated, as indicated 

 by the author as early as 1885. After years of experience, 

 it has proved impossible to speak of a preponderant species 

 or race from which any individual can be chosen. The 

 single type or species contains within itself so many varieties 

 which have come to development under the conditions existing 

 in practice, that a careful choice must be made from these. 

 A thorough study of a type by means of comparative experi- 

 ments will show which of the cultures is of the greatest value 

 in practice. 



As early as 1883, Koch's method of plate-culture was 

 tested by Hansen. He prepared a mixture of two species of 



