104 BACTERIA. 



hand, Saccharomyces Pastorianus II., kept under exactly the 

 same conditions, gives evidence of the commencement of 

 spore formation after seventy-seven hours ; z>., supposing that 

 the previous conditions have been the' same for the species 

 experimented upon. 



This single observation proved to be of enormous impor- 

 tance to brewers, for Hansen's pupils, Holm and Poulsen, 

 determined that it was possible by Hansen's method to show 

 that pure cultivated yeasts produced spores at a much later 

 date under similar conditions than did those saccha- 

 romyces that were capable of producing certain diseases in 

 beer when present in proportions of ^th of the yeasts intro- 

 duced ; and as it was found possible to determine by this 

 method the presence of wild yeasts when they occurred in 

 the proportion of ^Jo tri f the whole of the yeast used, it was 

 an easy matter to determine within a very short time whether 

 a yeast was fit to be used for brewing purposes or not ; in the 

 case in point, forty hours at the temperature of 25 C. was 

 sufficient for the purpose. It was found, indeed, that all 

 the bottom yeasts might be separated from one another by 

 making cultivations at a temperature of 15 C. and at 

 25 C. 



Having obtained these pure cultivations, and being so 

 successful with the study of the spores, Hansen turned his 

 attention to the other characters, by the use of which he was 

 able to separate in a still more definite manner the various 

 species of saccharomyces one from another. He subjected 

 to a most careful examination the films which appeared on 

 the surface of liquids that were undergoing fermentation. 

 It had been supposed that the Sacch. mycoderma was the 

 result of a yeast fermentation ; but Hansen found that the 

 saccharomyces film was something quite different from the 

 film formed by the Sacch. mycoderma, and he came to the 

 conclusion that film formation must be looked upon as a 

 phenomenon that may be met with under certain conditions 

 wherever micro-organisms are or have been growing. Here 

 again the presence of a plentiful supply of air is an absolute 

 necessity ; another essential condition is a state of perfect 

 rest of the surface of the fluid in which the process of yeast 

 growth and fermentation is going on. It is interesting to note 

 that when carbonic acid gas is passed through the fluid and is 

 allowed to accumulate on the surface these films are not formed, 



