90 VETERINARY BACTERIOLOGY 



lar weight of H 2 SO 4 is approximately 98, but there are two re- 

 placeable acid hydrogen atoms. Therefore, half this molecular 

 weight in grams, or 49 grams, of the H 2 S0 4 is made up to a liter 

 of solution and contains one gram of acid hydrogen. The same 

 principle is adopted in the preparation of a normal solution 

 of an alkali; in this case, however, it is necessary to divide the 

 molecular weight by the number of atoms of the base present^ 

 which will replace hydrogen. For example, the molecular weight 

 of NaOH is 40. It contains one atom only of sodium, and a normal 

 solution, therefore, contains 40 grams to the liter. Dry sodium car- 

 bonate (NajCOs) has a molecular weight of 106. Two atoms of sod- 

 ium are present ; therefore it is necessary to divide by two, so that 

 a normal solution of sodium carbonate contains 53 grams to the 

 liter of solution. It is evident that a given volume of a normal 

 solution of an acid will neutralize exactly an equal volume of a 

 normal alkali. 



It is customary to use indicators in the determination of the 

 acidity or alkalinity of a solution. Those most commonly used 

 are litmus, which is blue for alkaline and red for acid solutions, 

 and phenolphthalein, which is colorless with acid and red with 

 alkali. Phenolphthalein is so delicate an indicator that it is 

 sensitive to the presence of CO 2 in solution. It is, therefore, 

 necessary, whenever this indicator is used, to heat the solution 

 to boiling temperature in order to drive off any C0 2 which may 

 be present. It is customary to express the acidity or alkalinity 

 of a solution in terms of the amount of normal acid or alkali present 

 per 100 c.c. of solution. For example, if it is found that it requires 

 10 c.c. of the normal solution of alkali to neutralize 100 c.c. of a 

 given solution, we know that there is present in that solution the 

 equivalent of 10 c.c. of normal acid, and the reaction is expressed 

 as + 10. If the reaction is alkaline, the negative sign is used. 



Nature of Nutrients Required by Bacteria. It is found that 

 practically the same elements are necessary for the nutrition of 

 bacteria as are essential for higher plants and animals, but they 

 may be used in quite different proportions. 



It is particularly important that the disease-producing bacteria 

 be cultivated whenever possible. Cultivation outside the body is 

 quite necessary to a satisfactory proof of pathogenicity, to differ- 



