40 LABORATORY BACTERIOLOGY 
same number and kinds of tubes containing lactose, and the 
same containing saccharose. (The agar and the sugar-free 
bouillon necessary in the work of this exercise will be furnished 
by the instructor.) 
Read carefully the paragraphs in the text-books on the 
preparation and use of these media. 
54. Preparation of potato for a culture medium. Select 3 
medium-sized potatoes, thoroughly wash and rinse in boiled 
water, and cut out, with a cutter made for this purpose, cylin- 
ders from 3 to 4 cm. long (oblong rectangular pieces cut with 
a knife will do quite as well). Ordinarily 2 cylinders can be 
cut from each potato, These can be cut obliquely, giving 2 
pieces each. All of the skin must be removed. Wash the 
potato cylinders in cold running water for 5 minutes (@ 
longer time is preferable), place them in test tubes of the 
proper size (large or small according to size of cutter used), 
and add about 1 cc. of water to each tube. Sterilize them 
by discontinuous boiling or steaming for 20 minutes each day 
for 3 consecutive days. Wipe, label, and store in locker. 
55. Preparation of milk for a culture medium. It is better 
that the cream be removed from the milk before it is used. 
To do this the fresh milk (about 100 cc.) is placed in a beaker 
and set in the ice box for from 10 to 15 hours. Then care- 
fully remove the cream. It is well to filter the milk through 
a thin layer of absorbent cotton to remove any masses of 
cream that may be left. The reaction should be tested, and 
if strongly acid, should be rejected or made 1. 5% acid to 
phenol-phthalein by the addition of n/1o sodium hydrate. 
Distribute the skimmed milk in small test tubes (5 cc. in each) 
and sterilize by discontinuous steaming in-the same manner and 
for the same length of time as the potatoes. Wipe, label, and 
store the tubes in locker. “ 
56. Preparation of litmus milk for a culture medium. This 
is prepared the same as the milk medium, with the addition of 
enough of an aqueous solution of litmus to impart a decidedly 
