96 
Proceedings of Indiana Academy of Science. 
TABLE I. 
Decreas2s in Numbers of Bacterial Colonies on Plates with Increasing Length of Time of Incubation. 
erga) 
SPD DS NIN 0 HB 100 OTS G9 0 NS et bt et HD 0 et ee BD 
No. of Sets of 3 Plates Compared. 
_ 
Colony numbers between... . 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between... . 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between... .. 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Colony numbers between 
Jolony numbers between..... 
Colony numbers between 
Colony numbers between 
2,900 and 3,000 
1,700 and 1,800 
1,600 and 1,700 
1,500 and 1,600 
1,400 and 1,500 
1,300 and 1,400 
1,200 and 1,300 
.1,100 and 1,200 
1,000 and 1,100 
900 and 1,000 
800 
and 
and 
and 
and 
and 
and 
and 
5 and 
and 
5 and 
and 
and 
and 
and 
and 
and 
and 
and 
and 
and 
and 
900 
700 
600 
500 
400 
300 
250 
200 
175 
150 
125 
100 
90 
80 
70 
60 
50 
40 
30 
20 
10 
Average Counts 
3 days 7 days 
2,905. 2,045 
2,030 1,790 
1, 750 1,570 
1.600 1,050 
1,590 1,400 
1,456 1, 256 
1,380 1, 150 
1,240 960 
1,135 918 
1,070 1,010 
910 840 
852 778 
659 582 
541 480 
450 409 
340 327 
279 288 
222 220 
195 200 
163 168 
129 136 
115 125 
95 108 
83 91 
72 80 
65 71 
54 57 
43 53 
35 44 
22 30 
14 19 
7 12 
| 10 days 
Table II. 
1. Increases in counts resulted*from additional incubations when 
&. . 
less than 200 colonies were present after three days’ incu- 
bation. 
pa 
counts. 
o: 
4, 
The table shows the following: 
than 200 per plate. 
the higher dilutions under these conditions. 
Whether the counts increased or decreased, the counts after 
seven days’ incubation fall between the three- and ten-day 
Two hundred or more colonies gave unreliable results. 
The optimum number of colonies is probably much nearer 100 
The ratios between the number of colonies developing after ten days’ 
incubation of the 1—40,000 and 1—400,000 bacterial dilutions of soils 
taken at different times from differently cropped areas are given in 
In carrying out the dilutings and platings the lower dilutions 
were made and plated before the higher dilutions were prepared, since 
it is believed that multiplications of the organisms have little effect on 
