BACTERIA IN THE INDUSTRIES 209 



tion of food substances of all kinds. Weigh or measure a definite amount 

 of a well mixed average sample, mix, grind or triturate as may be necessary, 

 dilute as may be desired (1-5, i-io, i-ioo),and make the counts by means 

 of the hemacytometer. If the material is to be stained in order to make 

 bacterial counting possible, then place J^o cc. of the prepared and di- 

 luted material upon a clean slide and spread it out over an area of 10 sq. 

 cm. (2 cm. by 5 cm.), air dry, add alcohol, stain and make the counts by 

 means of the oil immersion objective, without the hemacytometer. The 

 pipette must have a free delivery and must be carefully graduated into 

 "tenths of one cc. This is essentially a modification of the Prescott and 

 methods as appUed to the direct milk count, differing in that larger amounts 

 are used (Mo cc. instead of Moo cc). It is impracticable to use the 

 smaller amount in most cases, and by using the larger amounts the source 

 of error is correspondingly less. The area of the field of view with the 

 oil immersion lens must be carefuUy determined. We will suppose 

 that the field of view is Ko sq. mm., the dilution used i-ioo, and the 

 average number of bacteria in one field of view is thirty, then the total 

 number of bacteria per cc. would be 1,500,000,000 (50 X 30 X 100 X 

 1,000 X 10 = 1,500,000,000). 



Those interested in a fuller discussion of the decomposition changes 

 in food substances and the microanalytical methods employed in the 

 examination of food substances should consult The Microbiology and 

 Microanalysis of Foods (P. Blakiston's Son and Company, 1920.) 



