BACTERIOLOGICAL TECHNIQUE. 217 



dishes, and then soaked for from twelve to eighteen 

 hours in running water; this removes excessive acidity; 

 they are then placed in test-tubes and sterilized by steam 

 on two consecutive days. 



Blood-serum and Ascitic Fluid with and without the 

 Addition of Bouillon. Blood-serum is used in the fluid 

 state, semi-solid, and firmly coagulated. It is used 

 alone, with 66 per cent, of bouillon and with 25 per 

 cent, of bouillon plus 1 per cent, of glucose. Ascitic, 

 pleuritic, and hydrocele fluids are also used alone, with 

 bouillon, or with nutrient agar. 



The Correction of the Reaction in Media. 



Formerly it was customary to use litmus-paper as 

 the indicator in neutralizing media, adding soda solu- 

 tion until the mixture turned the red litmus slightly 

 blue, and the blue litmus just a tinge less blue. This 

 is still the best method for those who are only going 

 to cultivate the common pathogenic bacteria for diag- 

 nostic purposes or for the development of toxin. Most 

 parasitic bacteria which grow at all on artificial culture 

 media develop best in them when they have a slightly 

 alkaline reaction to litmus. If a greater alkalinity is 

 desired a certain number of c.c. of normal soda solu- 

 tion can be added for each litre; if an acidity is desired, 

 normal hydrochloric acid solution is added. 



Many bacteriologists consider that litmus is not deli- 

 cate enough to be entirely satisfactory, especially when 

 experiments are to be reported or exactly repeated. 

 For these purposes phenolphthalein has been generally 

 selected. A little experience will show that different 

 indicators not only differ in delicacy, but that they 

 react differently to different substances. 



