160 LAURENCE F. FOSTER AND SAMUEL B. RANDALL 



SUMMARY AND CONCLUSIONS 



1. Broth (beef infusion, beef extract, ''bacto-beef") adjusted 

 to Ph values ranging from 5.0 to 9.0 undergoes a change in hydro- 

 gen-ion concentration upon autoclaving. This change is most 

 marked in media adjusted in the alkaline range (7.8-9.0), less 

 great in the acid range (5.0-6.2), and is usually inappreciable in 

 the neutral range (6.6-7.4). The maximum change is about 0.4 

 Ph and in the majority of cases is not over 0.2 Ph. 



2. The change is usually an increased acidity (decrease in Ph). 

 Decreases in acidity have been observed in a few instances but 

 these are exceptional. 



3. In media of the same composition the reaction changes are 

 not necessarily uniform in different experiments. 



4. Autoclaved broth undergoes changes in hydrogen-ion con- 

 centration upon standing; the degree of change is not influenced 

 by the environmental temperature within the limits, lO'C. (ice 

 chest) and 37°C. (incubator). 



5. The reaction changes upon standing, as in the case of auto- 

 claving, are most noticeable in the alkaline range, less marked in 

 the acid range, and least in the neutral range. Neutral media 

 usually do not change at all upon standing. 



6. The change upon standing is almost invariably in the direc- 

 tion of an increase in acidity. 



7. Broth adjusted to various Ph levels ranging from 5.0 to 9.0 

 and exposed in tubes plugged with cotton to an atmosphere of 

 CO2 for twenty-four hours shows marked alterations in reaction. 

 The change is always an increase in acidity, as would be expected. 

 The greatest change occurs in the alkaline range. 



8. Upon allowing broth adjusted to various Ph levels to stand 

 in a C02-free atmosphere the same reaction changes were noted 

 as in duplicate lots of broth allowed to stand in the air of the lab- 

 oratory. The increases in acidity exhibited by broth upon stand- 

 ing do not seem to be due to an absorption of atmospheric CO2. 



9. Reaction changes in media of Ph 5.2 to 9.2 do not appear to 

 stand in relation to changes in formol-titrating nitrogen. 



10. The possibility of an increase in acidity of broth through 

 the opening up of -COHN- groups during hydrolysis of the pro- 

 tein constituents suggested by Robertson remains a plausible one. 



