186 Report of the Department of Bacteriology of the 



Various media were tried in this work; but none ordinarily gave 

 higher counts than the soil-extract gelatin described in the earlier 

 articles. As a result, the figures chosen for publication are the 

 counts obtained upon that medium and are comparable with those 

 of the previous work. The composition of the medium is: 



Gelatin 12 per ct. 



Soil-extract 20 per ct. 



Dextrose 0.1 per ct. 



Reaction adjusted to 0.5 per ct. normal acid to phenolphthalein. 



In order to obtain the soil-extract, soil was heated for an hour at 

 one atmosphere pressure, then mixed with an equal weight of cold 

 water, allowed to stand over night, then boiled for half an hour and 

 filtered. 



The plates were incubated at a temperature of 17.5° to 18° C. 

 for seven days before counting. In averaging the counts, considera- 

 tion was taken of the fact that when over one hundred colonies 

 appeared on a plate, overcrowding generally prevented the develop- 

 ment of some of the bacteria. In such cases the greater dilution 

 almost invariably gave the higher count, and the lower dilution was 

 disregarded. Occasionally, however, the higher count was obtained 

 from the lower dilution even though there were over one hundred 

 colonies per plate, and then the eounts of both sets of plates were 

 averaged. If, on the other hand, there were less than one hundred 

 colonies per plate, the difference between the counts obtained from 

 the two dilutions was likely to be less than between those from 

 parallel plates of the same dilution; so in this case the counts obtained 

 from all the plates were averaged. In choosing the dilutions an 

 attempt was made to obtain between fifty and one hundred colonies 

 per plate in one or the other of the dilutions employed. 



results of pot work. 



This work was planned, as already mentioned, for two purposes: 

 to control soil moisture and to prevent the rise of bacteria from 

 lower depths. In 1912 two pots, one aerated and the other 

 unaerated, were prepared and tested occasionally during the following 

 winter. The soil was frozen for such short periods, however, that 

 the experiment was unsatisfactory, and it was repeated a second 

 year, with four pots instead of two. The soil in two of these pots 

 was aerated, in the other two unaerated. The two pots prepared 

 the first winter were left uncovered, so that their moisture content 

 rose and fell much the same as the field soil. During the second 

 winter the four pots were kept covered, and their moisture content 

 remained constant until the thaw in March, when melting snow 

 managed to get beneath the covers. 



The results of this work are given in Tables III and IV; while 

 the results for 1913-14 are also plotted in Graphs VI and VII. The 



