504 
Journal of Agricultural Research 
Vol. XXXI, No. 6 
of sodium thiosulphate and again with distilled water; finally with 
some sterile shake medium.” 7 Two soils were used for these tests. 
One gram in each case was vibrated with 2,000 c. c. diluent, then 
2 c. c. of this suspension again vibrated with 2,000 c. c., making a 
final dilution of 1: 2,000,000. The hand-shaking control plates of 
these soils gave 12 and 10 millions per gram soil, respectively. The 
analogous data for the vibrated suspensions were as follows: First 
test, 554 millions and 26 millions; second test, 47.5 millions and 41.2 
milhons. 
Although these counts are much lower than those made bv Whitr 
ties, they suffice to show that undoubtedly the error lay, in bis case, 
in the incomplete sterilization of the vibration cup. 
EFFICIENCY OF VIBRATION 
After having ascertained that the use of formaldehyde assures a 
satisfactory sterilization of the vibrator, the writers’ attention was 
turned to the effect of the vibration and to the value of the “ alkaline 
shake medium” recommended for making soil suspensions. The 
shake medium was prepared by diluting 50 c. c. of the stock solution 
(Na Cl 10 per cent : Na 2 C0 3 2.5 per cent) to 4 liters, which were ster¬ 
ilized. As inoculum two clay soils were chosen (Nos. 109 and 260). 
These had been in the laboratory for more than a week. Methods 
of making dilutions and plating were the same as in the preliminary 
experiments. The results of the nine tests are presented m Table 11. 
Table II .—Influence of vibration and “shake media” on total counts 
Suspension by— 
Dilution 
Sterile diluent 
Soil 109 
millions 
per gram 
Soil 260 
millions 
per gram 
TTsvnd shaking 
1 : 260,000 
1 :200,000 
1 :200,000 
“1 : 2,000,000 
«1: 2,000,000 
1 :200,000 
1 : 200,000 
1 : 260,000 
Tap water_ 
16.7 
13.0 
Vibrator 
_do_ 
18.8 
12.6 
Do 
Whi ties’ shake medium_ 
16.6 
11.8 
Do . . 
_do______ 
* 13.0 
“20.0 
Do. 
Whittles’ medium, plus 0.1 per cent gelatin_ 
«iao 
° 18.0 
Do. 
Whittles* shake medium, plus 0.1 per cent agar.. 
Whittles’ shake medium, plus 0.1 per cent gelatin 
Tap water_ 
15.9 
11.3 
Do. 
11.1 
10.6 
Hand shaking 
11.6 
10.1 
« Dilutions were made by placing 2 c. c. of the first dilutions back into the vibrator cup and repeating 
the vibration. The colonies were not numerous enough for accurate counting. 
These analyses extended over a period of several days and were 
made in the order given in the table. That there was very little 
change in the total number of microorganisms is shown by the close 
agreement of the hand-shaking control made at the beginning and at 
the end of the series. If the counts from the 1 :2,000,000 dilutions, 
which were too high, are excluded, the same agreement is evident 
throughout the table, whether a “shake medium” or tap water was 
used as diluent, and whether the suspension was made by the hand¬ 
shaking or by the vibrator method. Furthermore, the addition of 
a protective colloid to the suspension, such as 0.1 per cent gelatin 
or agar, gave no higher counts than when sterile tap water was used 
as diluent. 
i Nothing is said in these directions about the necessity of using not simply “distilled water,” but “ster¬ 
ilized distilled water.” 
