WHAT IS THE PURE YEAST OF PASTEUR? 141 



Experiment IV. The cultivation in the solution of cane sugar and 

 tartaric acid was in this case carried on for a month, and during this 

 time the culture was only transferred once to fresh solution, namely, 

 after a fortnight. The experiment was commenced with two flasks, and 

 into each of these were introduced the following species, namely, Sacch. 

 Pastorianus //., Sacch. Pastorianus ///., and Sacch. ellipsoideus II. 

 They were all obtained from vigorous growths which, however, had been 

 cultivated for three months on a nutrient gelatine containing fish decoc- 

 tion and cane sugar. At the conclusion of the experiment, Sacch. 

 ellipsoideus II. was found in both flasks and this alone. 



The methods of cultivation which were employed in order 

 to bring about the development of the species which had 

 survived the described treatment in the sugar solution, and 

 thus render their recognition possible, were such as, after 

 several years' experience, had been found to be favourable 

 to the growth of the species mentioned. If I had conducted 

 a still larger number of cultivations and had varied the 

 conditions in several other ways, I should probably have 

 had a chance of getting at least some of the species which, 

 under the circumstances, appeared to have perished. Where 

 the limit would be, cannot well be determined. Some of the 

 colonies in the gelatine cultures may also have contained 

 more than one species. In short, it is probable that more 

 species may have survived than were found. Those which 

 were detected must, therefore, in the first place be regarded 

 as the preponderating species. But even if we assume that 

 those flasks in which we found only one living species, only 

 contained this one, we have still as the main result the fact 

 that the method described gives us no certainty of obtaining 

 a pure culture. Out of nine flasks, three contained two species 

 each at the end of the experiments ; in two experiments, how- 

 ever, the treatment was carried on for three months. On 

 the other hand, it is not improbable that, if the experiment 

 had been carried further, all the species employed in one 

 experiment would have perished with the exception of 

 one ; this was the case with Sacch. ellipsoideus II. in the 

 first two experiments. But there is nothing here to guide 



