48 PURE CULTURE OF BACTERIA 



still acid repeat the above process until the medium is just 

 neutralized. 



4. Standard acidity. 



After neutralization add ic c.c. of normal hydrochloric acid 

 to every 1000 c.c. of medium, i.o per cent, free normal acid is 



now present, which leaves the 

 medium just sufficiently acid for 

 the growth of most bacteria. 



When working on bacteria com- 

 mon in soil add only 5 c.c. of 

 normal hydrochloric acid instead 

 of 10, as many of these refuse 

 to grow in the more acid 

 medium. 



It is usual to indicate these 

 standards of acidity by placing 

 the + sign before the number of 

 cubic centimetres of normal acid 

 used, e.g. + 10 and + 5 in the 

 cases mentioned. Where the 

 medium is neutral o may be used, 

 and in cases of alkaline media 

 the sign is employed before 

 the number of cubic centimetres 

 of normal caustic soda or sodium 

 FIG. 18. Hot-water funnel for filtration carbonate added to the medium 



of gelatine and agar media. 



after neutralization. 



5. To clarify. 



Cool the medium to below 40 C- and add slowly the white 

 of an egg well beaten up with a little water ; heat in the steam 

 sterilizer or a water bath, and then filter the hot solution through 

 filter paper moistened with boiling water into a sterilized flask : 

 use a hot-water funnel (Fig. 18) when filtering the medium. 



6. Sterilization. 



Take twenty or thirty clean sterilized test-tubes and pour into 



