REPORT OF COMMITTEE ON DESCRIPTIVE CHART 137 



ent degrees of acidity are listed in table 1, together with the ap- 

 proximate range of pH to which each corresponds. In the ab- 

 sence of accurate determinations, these degrees of acidity may 

 be recorded by the indefinite terms, ' 'weak," "moderate," "strong 

 and "very strong," or by the symbols + , + +, + + +, and 

 + + + +. If the student desires to record increase in alkalinity 

 in the same table on the chart, he can use the symbol for neu- 

 trality and — for an alkaline reaction. 



Gas production is ordinarily measured in percentage of gas in 

 the closed arm, and the ratio of H : C0 2 by means of absorption 

 with NaOH, using the methods described in laboratory manuals 

 (filling open arm with 4 per cent NaOH, allowing gas to enter 

 open arm, shaking and returning gas to closed arm). As this 

 method is far from accurate, it is recommended for provisional 

 use only. 



The fermentation test is ordinarily of no significance for 

 organisms of series III and IV because of their poor growth in 

 broth. Sometimes these organisms can be studied in some 

 other liquid medium in which they do give good growth; but 

 generally it is preferable to use agar slants. In such a case, use 

 a sugar agar containing brom cresol purple, china blue, or a mix- 

 ture of indicators as suggested on page 129. Increase in acidity 

 can be detected by fading of the purple color of the brom cresol 

 purple or by the appearance of blue if china blue is used. 

 Gas-production can usually be detected in agar cultures by the 

 presence of cracks and air bubbles, but as a test for gas, agar 

 slants are not as reliable as fermentation tubes. 



A more detailed discussion of hydrogen-ion concentration and 

 of methods for determining acid-production is given in the 1918 

 report of this committee, copies of which can be obtained from 

 the chairman. 



Milk. Acid production in milk can be detected by adding 

 brom cresol purple to the culture and comparing with the color 

 obtained by adding the same proportionate quantity of indica- 

 tor to sterile milk. (Brom thymol blue does not give satisfac- 

 tory results in milk.) Four degrees of acidity that can be simply 

 recognized in milk are listed in table 2. They correspond closely 



