132 Milk and Its Products 
steamed again for 20 minutes after the material has 
reached the temperature of steam. The third day they 
are steamed again for 20 minutes. 
Procedure for plating.—Wipe off the desk or table 
with 5 per cent carbolic acid solution. Shake thor- 
oughly twenty-five times, the sample of milk. Unless 
the bacterial count of the sample is nearly known, it 
is well to make three dilutions, with two plates per 
dilution (six plates in all). The dilution should be 
large enough so that not more than one or two hun- 
dred colonies will develop on the petri dish. Thus, in 
plating fresh milk, a dilution of 1:100, 1:200 and 
1: 1000 may be used. To make the 1: 1000 dilution, 
measure out with a sterile pipette (care should be taken 
in drawing out the pipette from the case that the 
point touches nothing but the milk, and, if many pi- 
pettes are kept in a metal case, each pipette should be 
passed through a flame before using it) 1 ¢. e. of 
milk, and put it in a sterile bottle containing 99 c. ¢. 
of the sterilized water. Shake this thoroughly. With 
another pipette, place 1 c. c. of the solution in a 
petri dish, being careful to raise the cover of the petri 
dish only high,enough to introduce the mouth of the 
pipette. Five-tenths of a cubic centimeter of this solu- 
tion would give a dilution of 1: 200, while .1 would be 
1:1,000. If higher dilutions are needed, more bottles 
may be used; for instance, if a dilution of 1 : 1,000,000 
is wanted, arrange three bottles each with 99 c. ec. of 
water. Place 1c. ec. of milk in the first and shake thor- 
oughly. Takelc. ec. of this dilution; place in the second 
bottle and again shake thoroughly, Take 1 <¢. ¢c. of this 
