36 MICRO-ORGANISMS AND FERMENTATION. 



requirements are generally satisfied by the wort employed in 

 breweries. 



If it is found that the determinate volume contains too many 

 cells to be counted with certainty, the liquid must be diluted. 

 This may be advisable for other reasons, partly to prevent the 

 formation of froth, which may otherwise form abundantly from 

 the violent agitation, and partly to isolate the single cells 

 which frequently cluster as colonies in the wort, and are not 

 always separated by shaking, and, finally, in order to bring 

 about a discontinuation of the fermentation and multiplication 

 of the yeast-cells at the beginning of the experiment. 



HANSEN found that dilute sulphuric acid (1 : 10) on the 

 whole answers these requirements ; hydrochloric acid, ammonia 

 and caustic soda may also be used, but they are not so good. 

 If very great dilution is required, distilled water may be added, 

 after the addition of one to two volumes of dilute sulphuric 

 acid. 



When the different volumes of liquid are measured with 

 accuracy, and particular care taken that the cells are thoroughly 

 distributed by vigorous and prolonged shaking, the determin- 

 ation can be made with great accuracy. Two similar dilutions 

 must always be made, and samples taken from each for count- 

 ing. As a matter of course, experiments must also be made to 

 determine the number of the small squares whose cell contents 

 must be counted in order to arrive at a true average. Such a 

 counting and determination of the average numbers is con- 

 tinued until the number finally obtained is found to have no 

 further influence on the average value. The number of count- 

 ings necessary, and the accuracy generally, depend on the 

 experience and care of the observer. HANSEN found that, as a 

 general rule, it was sufficient to count the cells in 48 to 64 

 small squares. 



