300 PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE. 



In investigations with milk and food products large variations in results 

 have been credited to differences in manner of shaking up the samples and 

 bacterial dilutions. Dr. H. W. Conn,'' found within the last two years that 

 variations occurring between reports from different collaborators on the same 

 sample of milk might be due to variations in the way in which different 

 workers carried out given directions for shaking. The rubber stoppers are 

 put in the first set of bottles so that they may be shaken more vigorously, 

 as it is here that colonies ought to broken up. The number of times specified 

 and the lengths of time given are considered sufficient to overcome variations 

 in carrjdng out the technic of shaking the bacterial dilutions. 



Plating 

 A one cc aliquot of the proper bacterial dilution is put directly into the 

 petri plate before the media is added. This is so that all the bacteria in the 

 aliquot may have a possible chance to develop into colonies. Care should be 

 taken to rotate the dish sufficiently to have the bacteria evenly distributed 

 through the media. It has l)een our experience to have the bacteria in one 

 ce aliquots evenly distributed throughout the media when the bacteria in 

 0.1 cc aliquots were clumped together. One cc aliquots are advised both to 

 increase the accuracy of aliquoting and to insure more representative ali- 

 quots. 



Incubator 



We do not feel justified in discussing incubators for this laboratory has 

 an inside room over the vaults where the temperature does not vary over 

 four degrees Centigrade in the course of a year. 



Reports 

 Reports are always made of the number of bacteria or th(> amount of 

 material per gram of dry soil. 



Physiological Tests and Media 



Three methods of studying the activities of the soil bacteria have been 

 mentioned together with certain solutions for starting these tests. These 

 methods of determining the physiological activities are not necessarily 

 recommended as we have not studied them in great detail or compared them 

 extensively with other methods. They are given to emphasize the advisability 

 of weighing out the aliquots of soil to be used for physiological tests at the 

 same time the aliquot from which bacterial dilutions are to he made is weighed, 

 out. 



The media used in the laboratory has ])een omitted because the technic 

 is the same whatever the agar media used. 



Note. Glass stoppered bottles such as are used in milk laboratories are 

 not used for dilutioii bottles. They are not even used where rubb(>r stoppers 



