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 i.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 i.e., from one tube to another, by means of 

 the platinum wire having a loop at its extremity. 



