LABORATORY AND TEACHING METHODS 



617 



LESSON 20 



Counting of Yeast Cells, Fungous Spores and Bacteria. — In many cases the cells 

 are in a liquid which is inclined to form froth when shaken, hence the liquid can be 

 treated with dilute sulphuric acid (i part concentrated sulphuric acid and 10 parts 

 water). This prevents aggregations of the cells and also furnishes in addition a 

 liquid in which cells do not sink to the bottom too quickly, an important point, when 

 single drops are taken out for counting purposes. 



In counting, the counting chamber is employed. Thoma's hsematimeter consists 

 of a glass slip on which a, cover-glass is fastened which has a circular hole in the 



Fig. 217. — A, Squared cover glass used in counting; B, Jorgensen's squared 

 cover glass; C, moist chamber, sectional view; D, moist chamber with Jorgensen's 

 squared cover. (A and B, after Klocker; C, original; D, after Jorgensen.) 



middle and is 0.2 mm. thick (Fig. 218). A circular cover-glass, o.i mm. thick, is 

 fitted centrally in this hole and is also fastened to the glass slip; thus an annular 

 space is formed. In the middle of the cover-slip two sets of twenty-one parallel 

 lines are etched which cut each other at right angles; there are thus formed a large 

 square with a side of i mm. and small squares with a side of 0.05 mm. The drop of 

 liquid taken up by a pipette is examined on this square and enclosed by the cover- 

 glass, the depth of the liquid layer thus formed amounting to o.i mm. (Fig. 218). 



Thoma's Hcematimeter. — After the test-tube with the average sample and the 

 H2SO4 has been subjected to a prolonged and vigorous shaking, a sample is taken out 

 and e,\aminedjas above. 



