34 MICRO-ORGANISMS AND FERMENTATION. 



system of squares of known size, engraved on the object- 

 glass itself at the bottom of the cavity. This also improves 

 the microscopic definition of the cells which are on the 

 bottom of the chamber. 



When it is merely desired to determine the rapidity with 

 which the cells multiply, or to make repeated observations of 

 the number of cells in the same volume, it is quite super- 

 fluous to determine the size of this volume ; it is then only 

 necessary to work always with the same volume. 



It is always necessary that the sample taken should be a 

 fair average one. In most cases it must be diluted and 

 thoroughly agitated for a long time, in order to obtain an 

 equal distribution of the cells ; the specific gravity of the 

 liquid must also be such that it will allow the cells to remain 

 suspended in it for a short time. A small drop is then 

 withdrawn in a capillary tube, transferred to the counting 

 apparatus, and covered with the cover-glass. The apparatus 

 is now allowed to remain at rest for some time, in order that 

 the cells may settle to the bottom of the enclosed space, and 

 on this account the specific gravity of the liquid must not 

 be greater than will allow this to take place in a convenient 

 time. Both these 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 also be advisable for other reasons, partly 

 to prevent the formation of froth, which otherwise will 

 generally form abundantly from the violent agitation, and 

 partly to isolate the single cells which are frequently clustered 

 in colonies or large masses 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 



