278 MORRIS S. FINE 
Fig. 5 Bacterial infusions, average ‘phenolphthalein acidity.’ Ordinates 
represent number of cubic centimeters of 0.01 N NaOH per 100 ce. of infusion. 
Upper curve, coli. Lower curve, subtilis. 
form acidity is of course attained when the rate of diffusion of hay 
constituents balances that of their solution. The present study 
contains many data illustrative of the above explanation. 
Peters is of the opinion that the deeper layers of the infusion 
owe their higher acidity to greater concentrations of COs. ‘‘Ow- 
ing to the slowness of diffusion, each upper layer of the liquid 
protects the adjacent lower layer from rapid interchange with the 
air and enables the deeper layer to maintain a higher acidity.” 
This factor is, I believe, probably only of secondary importance. 
The greater acidity in the lower regions of the hay infusion is 
to be attributed primarily to their greater concentration of acid- 
yielding hay constituents, as explained above. Peters’ explana- 
tion would not entirely account for the fact that infusions of the 
B series show uniform acidity throughout the cultures, while the 
acidity differences in an infusion of series C is very great. 
In the early history, at least of the typical members of series 
C, the increase in acidity is not uniform with increase in depth. 
Until the immediate region of the hay is reached, the acidity may 
be practically uniform. To illustrate this: on the fifth day a 
sample taken from just above the hay in infusion C-2” titrated 
exactly the same as a sample from the top, while the difference 
between the acidity at the top and extreme bottom was 58. On 
if Loe Cit... pemac: 12 See table 4. 
