XXIV] SPICES AND OTHER CONDIMENTS 325 



It is of the utmost importance that the drop be mixed thoroughly and spread 

 evenly, otherwise the insohil)le matter, and consequently the molds, are most abun- 

 dant at the center of the drop. Squeezing out of the more liquid portions around 

 the margin must be avoided. In a satisfactory mount Newton's rings should be 

 apparent when finally mounted and none of the liquid should be drawn acro.s3 the 

 moat and under the cover-glass. 



Place the slide under the microscope and examine with about 90 diameters and 

 with such adjustment that each field of view represents approximately 1.5 sq. mm. 

 of area on the mount. 



Observe each field as to the presence or absence of mold filaments and note the 

 result as positive or negative. Examine at least 50 fields, prepared from 2 or more 

 mounts. No field should be considered positive unless the aggregate length of the 

 filaments present exceeds approximately one sixth the diameter of the field. Cal- 

 culate the proportion of positive fields from the results of the examination of all 

 the observed fields and report as percentage of fields containing mold filaments. 



51 YEASTS AND SPORES.— TENTATIVE. 



Fill a graduated cylinder with water to the 20 cc. mark, and then add the sample 

 till the level of the mixture reaches the 30 cc. mark. Close the graduate, or pour 

 the contents into an Erlenmeyer flask, and shake the mixture vigorously 15-20 sec- 

 onds. To facilitate thorough mixing the mixture should not fill more than three 

 fourths of the container in which the shaking is performed. For tomato sauce or 

 pastes, or products running very high in the number of organisms, or of heavy con- 

 sistency, 80 cc. of water should be used with 10 cc. or 10 grams of the sample. 

 In the case of exceptionally thick or dry pastes, it may be necessary to make an 

 even greater dilution. 



Pour the mixture into a beaker. Thoroughly clean the Thoma-Zeiss counting 

 cell so as to give good Newton's rings. Stir thoroughly the contents of the beaker 

 with a scalpel or knife blade, and then, after allowing to stand 3-5 seconds, remove 

 a small drop and place upon the central disk of the Thoma-Zeiss counting cell and 

 cover immediately with the cover-glass, observing the same precautions in mount- 

 ing the sample as given under 50. Allow the slide to stand not less than 10 min- 

 utes before beginning to make the count. Make the count with a magnification 

 of about 180 (8 mm. apochromatic objective with the X6 compensating ocular). 



Count the number of yeasts and spores on one half of the ruled squares on the 

 disk (this amounts to counting the number in S of the blocks, each of which con- 

 tains 25 of the small ruled squares) . The total number thus obtained equals the num- 

 ber of organisms in l/60cmm. if a dilution of 1 part of the sample with 2 parts of water 

 is used. If a dilution of 1 part of the sample with 8 parts of water is used, 

 the number must be multiplied by 3. In making the counts, the analyst should 

 avoid counting an organism twice when it rests on a boundary line between 2 adja- 

 cent squares. 



52 BACTERIA.-TENTATIVE. 



Estimate the bacteria from the mounted sample, used in 51, but allow the sample 

 to stand not less than 15 minutes after mounting before counting. Use a magnifi- 

 cation of about 500 (8 mm. apochromatic objective and X18 compensating ocular). 

 Because of the somewhat clearer definition obtained with the X12 compensating 

 ocular, some prefer it to the X18, though the magnification is correspondingly less, 

 being about 375. Count and record the number of bacteria in a small area consist- 

 ing of 5 of the small sized squares. Move the slide to another portion of the field 



