CULTURES IN SOLID MEDIA. 77 



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 48) 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. 



