198 BIOLOGY AND TECHNIQUE 



tivated in a strong pepton solution to which 0.1 c.c. of a one per 

 cent solution of ferric tartrate or lead acetate has been added. The 

 addition of these substances gives rise to a yellowish precipitate 

 in the bottom of the tubes. If, on subsequent inoculation, the 

 bacteria produce H 2 S, this precipitate will turn black. The solution 

 recommended by Pake for this test is prepared as follows : 



1. Weigh out 30 grams of pepton and emulsify in 200 c.c. of tap water 

 at 60 C. 



2. Wash into a liter flask with 80 c.c. tap water. 



3. Add sodium chlorid 5 grams and sodium phosphate 3 grams. 



4. Heat at 100 C. for 30 minutes, to dissolve pepton. 



5. Filter through paper. 



6. Fill into tubes, 10 c.c. each, and to each tube add 0.1 c.c. of a one 

 per cent solution of ferric tartrate or lead acetate. These solutions should 

 be neutral. 



7. Sterilize. 1 



ACCURATE QUANTITAVE GAS ANALYSES of bacterial cultures can be 

 made only by the more complicated methods used in chemical labora- 

 tories for quantitative gas analysis. The gas, in such cases, is 

 collected in a bell jar mounted over mercury, and subjected to 

 analysis by the usual method described in works on analytical 

 chemistry. 



Acid and Alkali Formation by Bacteria. Many bacteria produce 

 acid or alkaline reactions in culture media, their activity in this 

 respect depending to a large extent upon the nature of the nutrient 

 material. Many organisms which on carbohydrate media produce 

 acid will give rise to alkali if cultivated upon media containing only 

 proteins. 



Information as to the production of acid or alkali can be obtained 

 by the addition of one of a variety of indicators to neutral media. 

 The indicators most often employed for this purpose are litmus, 

 neutral red, China blue and the so-called Andrade indicator. An- 

 drade consists of 100 c.c. of a 0.5% aqueous solution of acid fuchsin 

 to which 16 c.c. of accurately normal NaOH has been added. 

 Changes in the color of these indicators show whether acids or 

 alkalis have been produced. 



Great help in differentiation is obtained by adding chemically 

 pure carbohydrates to media to which litmus has been added, and 



1 Quoted from Eyre, ' ' Bact. Technique, ' ' Phila., 1903. 



