44 METHODS OF CULTIVATION OF BACTERIA. 



above procedure of dealing with the reaction of a medium is 

 sufficient for ordinary work it is important to have a more 

 exact method for making media to be used in growing 

 organisms, the growth characteristics of which are to be 

 described for systematic purposes. Such a method should 

 also be used in studying the changes in reaction produced 

 in a medium by the growth of bacteria. It, however, 

 involves considerable difficulty, and should not be under- 

 taken by the beginner. It entails the preparation of 

 solutions of acid and alkali which may be used for deter- 

 mining the original reaction of the medium, and for 

 accurately making it of a definite degree of alkalinity. 

 Normal l and decinormal solutions of sodium hydrate and 

 hydrochloric acid are used. 



Preparation of Standard Solutions. These cannot be accurately 

 made by direct methods. Sodium hydrate takes up water from the 

 air while being weighed, and the accurate estimation of the strength 

 of hydrochloric acid from the specific gravity requires the most delicate 

 chemical manipulations. To prepare these solutions the first require- 

 ment is a deci-normal solution of silver nitrate. This salt can be most 

 accurately weighed in a chemical balance. Its molecular weight is 170, 

 therefore a deci-normal solution will have 17 grs. dissolved in one 

 litre of distilled water. From this a normal solution of hydrochloric 

 acid can be derived by titration, potassium chromate being used as 

 an indicator. From this a deci-normal acid solution can be made. 

 Corresponding standard (normal and deci-normal) solutions of soda 

 (NaOH) are prepared by titrating against the acid solutions, phenol- 

 phthalein being here used as the indicator. 



The above standard solutions having been obtained, the 

 reaction of a given medium is found by titration. Since 

 the original reaction of meat extract ought to be acid, as 



1 A "normal" solution of any salt is prepared by dissolving an 

 " equivalent " weight in grammes of that salt in a litre of distilled water. 

 If the metal of the salt be monovalent, i.e. , if it be replaceable in a com- 

 pound by one atom of hydrogen (e.g. sodium), an equivalent is the mole- 

 cular weight in grammes. In the case of NaCl, it would be 58.5 grammes 

 (atomic weight of Na = 23, of 1 = 35.5). If the metal be bivalent, i.e., 

 requiring two atoms of H for its replacement in a compound (e.g., cal- 

 cium), an equivalent is the molecular weight in grammes divided by two. 

 Thus in the case of CaCl 2 an equivalent would be 55.5 grammes (atomic 

 weight of Ca = 40, of Cl 2 = 7i). 



