THE DILUTION METHOD. 



125 



0.5 m.m. thick, then various portions of the liquid, which has a 

 uniform thickness of o.i m.m., can be examined under the micro- 

 scope for the number of germs present therein. 



A number (ten to fifty) of the square divisions are counted and 

 the mean of the resulting figures is taken. This being denoted as 



M 



M, the germ content of the liquid will then be = 4000 M 



per cubic millimetre. 



In order to arrest the movement of motile forms or prevent 

 the multiplication of rapid-growing cells (e.g. yeast), a portion of 

 the sample, well shaken up, is previously mixed with an equal 

 volume of 10 per cent, sulphuric acid, which will kill the organisms. 

 This dilution must be taken into account in calculating the germ 



A 



s 



I FIG. 32. Counting Chamber. Nat. size. Description in text. 



content from the numbers found in the counting ; so that M must 

 be multiplied by 8000 instead of 4000. 



Assuming the germ content to have been ascertained in the 

 way described above in the bacterial mixture, the different species 

 in which are being isolated ; then, the number of cells present per 

 unit of space being known, a portion of the sample but not that 

 diluted with acid must be thinned down with sterilised water 

 to such an extent that only one cell is present in two to five drops. 

 One drop of this diluted liquid is then placed in each of a series 

 of flasks containing a sterile nutrient medium, which flasks are 

 subsequently kept at a suitable temperature, whereupon some of 

 them will, after a while, exhibit signs of development ; these will 

 constitute the wished-for pure cultures. They are, however, not 

 unconditionally reliable, since it not infrequently afterwards be- 

 comes evident that, in despite of calculation, some of the flasks 

 contained more than one germ. By this method, generally known 



