1918] DAIEY FARMING DAIEYING. 75 



surface colonies which were very much larger than those which grew under the 

 surface and so only deep colonies were measured. In every case the size of the 

 colonies decreased with an increase in acidity beyond +0.5, while in all except 

 B. lactis acidi the neutral reaction gave slightly larger colonies than did the 

 +0.5 agar. The streptococcus and micrococcus failed to develop on agar with a 

 reaction of +1.5. With B. lactis acidi there was a much greater difference in 

 the size of the surface colonies on the different reactions than there was in the 

 case of deep colonies. By using the surface colonies of B. lactis acidi the fol- 

 lowing diameters were obtained: Neutral — 1,280 microns, +0.5 — 1,320 microns, 

 +1 — 630 microns, and +1..5 — 200 microns. The relative sizes of the colonies 

 on agars of different reactions are represented graphically. It is recommended 

 that +0.5 be adopted as the standard reaction of agar for the bacterial analysis 

 of milk. 



The determination of bacteria in ice cream, S. H. Ayers and W. T. John- 

 son, JB. (TJ. S. Dept. Agr. Bui. 563 (1917), pp. 16, fig. J).— After calUng atten- 

 tion to the difficulty of making accurate bacteriological analyses, the authors 

 report results of experiments, the object of which was to throw light upon the 

 evenness of disti*ibution of bacteria in ice cream. 



The results indicate that " bacteria in commercial ice cream are distributed 

 quite evenly, and that an analysis of one sample from a gallon of ice cream 

 gives results which will hold for any other similar sample from the same gallon. 

 Storage of ice cream for 11 days in a commercial ice cream cabinet or in a 

 hardening room for a period of two months did not seem to cause an uneven 

 distribution of bacteria. In a series of from five to ten samples taken directly 

 from a large commercial freezer, the bacterial counts on each sample checked 

 \vithin the usual limits of error of bacterial analyses." 



In a comparison of two methods of incubation it was found that incubation 

 at 37° C. for 48 hours did not give counts which showed greater variation than 

 those obtained by incubation at 30° for five days. It is noted, however, that 

 the counts obtained by the latter method of incubation were practically 

 double those obtained by the former. 



A study was also made of the question as to which dilution will give the 

 more accurate count. The results show that when dilutions were such that 

 about 200 colonies were present on the plates a lower variation between counts 

 of samples of ice ci-eam was found than when there were 50 or fewer colonies 

 per plate. 



Variations between duplicate counts from the same sample and the same 

 dilution ranged from 7 to 26.6 per cent. Among duplicate plates in the exami- 

 nation of other samples of ice cream a variation as high as 41 per cent was 

 found. " This must be remembered in connection with the fact that the varia- 

 tion found in our experiments between average counts of different samples of 

 Ice cream from the same gallon lot ranged, generally speaking, between 20 and 

 SO per cent. To this variation between duplicate plates or a series of plates 

 from tlie same dilution must be added the error introduced in removing 1 cc. 

 portions of ice cream from different samples." 



In discussing the interpretation of bacterial counts, it is stated that differ- 

 eiices in the number of bacteria per cubic centimeter should never be consid- 

 wed except in relation to the total count of each sample. 



Studies on the production of sanitary milk, J. M. Shebman (Pennsylvania 

 Sta. Rpt. 1915, pp. 299-305). — Brief reports are made of studies under the 

 following headings : 



I. /?0OT3 bacteriological tests of the milk clarifier. — The milk used in the pre- 

 liminary tests here reported came from the college herd and was produced 



