COUNTING YEAST CELLS. 55 



driven out. The stopper is sealed with paraffin wax, and if 

 gelatine or agar is being used the test-tube is rotated round 

 its longer axis until the material has solidified. The bacteria 

 develop slowly on the inner surface of the glass. 



The cultivation can also be carried out by utilising a sub- 

 stance that will absorb oxygen e.g., pyrogallic acid (1 gramme 

 of the dry reagent in 10 c.c. of a one-tenth normal potash 

 solution). To carry this out the open test-tube or plate- 

 culture is placed in a larger air-tight test tube or vessel con- 

 taining the reagent. The absorption of oxygen goes on slowly, 

 and requires 24 hours or more. The culture may also be 

 covered with paraffin, vaseline, oil, plates of mica, etc. 



Reducing substances like grape sugar, especially in an 

 alkaline solution, or minute quantities of formic acid or sodium 

 indigo sulphonate may be added to the nutritive substance, 

 in order to favour the growth of anaerobes. 



If it is wished to ascertain with certainty, in using one 

 of these processes, when all oxygen has disappeared, a con- 

 centrated alcoholic solution of methylene blue may be used 

 as an indicator. A few drops are added to the nutrient, 

 and as soon as the last trace of oxygen is absorbed or 

 removed the indicator will be entirely decolourised. The 

 same applies to the addition of indigo carmine (neutral 

 sodium indigo sulphonate). 



8. Counting the Yeast Cells. 



The multiplying capacity of the yeast cells can be estimated 

 by directly counting the cells that are present in a given 

 volume of the liquid at different stages of the fermentation. 

 Experiments on these lines have been undertaken especially 

 by Delbriick, Durst, Hansen, Hayduck, and Pedersen, whilst 

 Fitz has applied the method of counting to bacteria. 



The counting is performed by means of an apparatus 

 constructed by Hayem and Nachet, and by C. Zeiss (Fig. 13), 

 which was first employed for counting the corpuscles of blood 

 (hence termed hcematimeter). Panum was the first to employ 

 this apparatus for counting micro-organisms, in order to 



