404 MILK AND MILK PRODUCTS 
800. oq x90 = 21400,000 
5) yan 120,000 bacteria per cubic centimeter, where 
600 = number of colonies counted; 
20= number of fields counted; 
20 = dilution—1-20; 
200 = microscope factor. 
Frost (1915, 1917) has secured a satisfactory agreement between his 
method and the standard plate method. In his second communication, 
this author reports some results secured with his method during a prac- 
tical test on the Boston milk supply. The table which is presented in 
this book shows that Frost’s microscopic method gives results which 
are different than those obtained on the standard agar plate. 
Allen’s Microscopic Method for Counting Bacteria in Milk. Allen 
(1918) has described his method as based on the fact that a water sus- 
pension of aluminum hydroxide readily collects bacteria in milk. This 
may then be thrown down in one end of a centrifuge tube leaving the 
normal milk constituents suspended. Allen has described his tech- 
nique as follows: Preparation of Hydroxide Suspension. Mix equal 
parts of N/20 AIK(SO4)2—12H20 and N/20 sodium hydroxide. After 
precipitation 1s complete, wash the precipitate thoroughly by decanta- 
tion using scrupulously distilled water, keeping in mind that any dirt 
in the wash water will be taken up by the aluminum hydroxide and will 
appear in the film under the microscope. Make up to one-half the 
original volume and continue to dilute until a proper amount of alu- 
minum hydroxide is obtained after centrifuging. Procedure. 1. 
Add to the sample of milk to be analyzed enough of the washed alu- 
minum hydroxide precipitate suspension so that it becomes 20 per cent 
of the mixture. Shake thoroughly for several minutes. 
2. Add 2.5 ¢.c. of the above mixture to a centrifuge tube holding 
this amount when stoppered. (Use plain centrifuge tube open at both 
ends for stoppers.) 
3. Centrifuge for fifteen minutes at 5000 r.p.m. in a centrifuge of 
10-in. diameter or give equivalent centrifugalization. 
4. Pull stopper at cream end of the tube and remove the cream with 
a needle allowing the milk to run out. Then carefully pull the stopper 
at the other end of the tube and transfer the plug of aluminum hydroxide 
to a clean glass slide laying over glass or cardboard ruled off in square 
centimeters. By use of a needle or loop grind up the precipitate on the 
