150 STERILIZATION AND DISINFECTION 
dilutions than 1: 500 are to be made a 1 per cent solution is made 
from the 5 per cent solution and the higher dilutions made from 
this. 
We have adopted the following scale for making dilutions: 
For dilutions up to 1: 70, increase or decrease by a difference of 5 
(i.e., 5 parts of water). 
From 1: 70 to 1: 60 by a difference of 10. 
From 1 : 160 to 1 : 200 by a difference of 20. 
From 1 : 200 to 1 : 400 by a difference of 25. 
From 1 : 400 to 1 : 900 by a difference of 50. 
From 1: 900 to 1: 1800 by a difference of 100. 
From 1: 1800 to 1 : 3200 by a difference of 200. 
And so on if higher dilutions are necessary. 
It is important that the cylinders used for making the dilutions be 
correctly graduated, as we have found disregard of this factor an im- 
portant source of error. It is preferable to use standardized cylinders 
and pipettes, and we recommend that they be used whenever possible. 
They, of course, should be perfectly clean. For making the dilutions 
in accordance with the above scheme we have found the following tables 
of much service: 
Seeding Tubes. The seeding tubes are glass test tubes 1 in. in 
diameter and about 3 ins. long, with round bottoms. In order to 
measure the disinfectant into them they are placed in a suitable wooden 
stand to receive them. We found it convenient to use a wooden block 
containing 6 rows of 15 holes each for the disinfectant to be tested and 
a separate stand for the phenol controls. The tubes are placed in the 
stand and each marked with the strength of dilution it is to 
contain. 
Starting with the lowest dilution (i.e., the strongest), the cylinder 
is shaken, then 5c.c. are measured into the tubes marked to receive 
that strength, using a 5-c.c. delivery pipette. In order to economize 
glassware, the same pipette is used for measuring out the next dilution, 
first blowing out as much of the remaining liquid as possible, then 
drawing a pipetteful of the next dilution to be used and discarding that; 
then filling the pipette a second time which is emptied into the seeding 
tube. 
The measuring out being completed, the tubes are placed in the 
water bath and allowed to stand a few minutes in order that the dis- 
infectant solution may reach the standard temperature. We have 
