66 A MANUAL OF BACTERIOLOGY 



hydrochloric acid is then added. The reaction generally 

 adopted corresponds to 10 c.c. or 15 c.c. of normal hydro- 

 chloric acid per litre of medium after neutralisation. 

 Even with this addition, the medium is still alkaline to 

 litmus. The reaction is designated by the figure corre- 

 sponding to the number of cubic centimetres of normal 

 acid (or alkali) per litre by which the reaction differs 

 from the neutral point to phenolphthalein, preceded by the 

 plus sign if acid, by the minus sign if alkaline. Thus 

 a medium + 10 means that the medium is acid to 

 phenolphthalein to the extent of 10 c.c. of normal acid 

 per litre. (The Americans formerly reckoned the reaction 

 per 100 c.c. of medium, so that 4- TO on their scale 

 corresponds to -f 10 on ours.) Whatever the reaction 

 adopted, it should be stated. Various methods are 

 adopted to obtain the final reaction ; the American 

 Committee recommend first neutralising and then adding 

 sufficient acid (or alkali) ; Eyre, having calculated the total 

 acidity, adds only sufficient alkali to reduce, the acidity 

 to the required point. Nutrient gelatin and agar are 

 standardised in the same manner after preparation, being 

 kept fluid by heating during the process. 



It is, however, becoming old-fashioned to speak of 

 a reaction as being acid or alkaline ! Reaction is now 

 described in terms of " hydrogen-ion concentration," 

 designated P H . According to the ionic theory, every 

 salt, acid, or base in aqueous solution breaks up to 

 some extent into atoms or groups of atoms carrying 

 an electric charge ; these are known as " ions." Thus 

 sodium chloride in solution may be regarded as existing 

 as NaCl, Na, and Cl. Even pure water itself dissociates 

 to a small extent, so that there are present H 2 0, H and 

 OH. H-ions bear a positive electric charge, OH-ions a 

 negative charge. Free H-ions are acid, free OH-ions are 

 basic. In pure water both are present in equal amount, 



