CULTURES IN SOLID MEDIA. 79 
thorough sterilization by steam the culture medium is ready for use. 
In the same way other vegetables, or bread, etc., may be used for 
special purposes, and especially for cultures of the mould fungi. 
Potatoes usually have a slightly acid reaction, and on this ac- 
count certain bacteria will not grow upon them. This acid reaction 
is not constant and differs in degree, and as a result we may have 
decided differences in the growth of the same species upon different 
potatoes. To overcome this objection the writer has sometimes neu- 
tralized the cones.of potato in test tubes (see Fig. 21, page 49) by 
first boiling them in water containing a little carbonate of soda. 
The liquid is poured off after they have been in the steam sterilizer 
for half an hour, and they are returned for sterilization. 
Salomonson’s Method of cultivation in capillary tubes has a his- 
torical value only since the introduction of Koch’s plate method. 
The following modifications of Koch’s plate cultures have recently 
been introduced: 
Kruse (1894) pours the liquefied gelatin or agar into Petri dishes, 
and after it is solidified brushes the surface with a sterilized camel’s- 
hair brush which has been dipped into water containing in suspen- 
sion—properly diluted—the bacteria to be studied. By this procedure 
surface colonies only are obtained. Von Freudenreich (1894) prefers 
to pour the contents of the test tube upon the surface of the sterile 
medium, in Petri dishes. The fluid is allowed to run off by placing 
the Petri dish in a vertical position, and this is subsequently placed in 
the incubating oven in an inverted position—7.e., with cover below. 
To obtain satisfactory plates with well-separated, superficial colonies 
it may be necessary to use two or three dilutions, made in sterilized 
water in the usual way—17.e., from one tube to another, by means of 
the platinum wire having a loop at its extremity. 
