hy k. gre1c4-smith. 171 



The Influence of Pveaction upon Bacterial Growth. 



An extract of the same vegetable-mould as in Expt. xix. was 

 prepared, and the acidity was determined in the following 

 manner. Twenty-five c.c were pipetted into a beaker, an equal 

 volume of distilled water was added, and the beaker was covered. 

 It was boiled for ten minutes to expel the carbon dioxide, and 

 rapidly cooled. Three drops of phenolphthalein were added, and 

 N/100 soda was run in until a difference in the tint was manifest. 

 An end-point was not aimed at; just the difference in the tint 

 from a yellowish to a faint brown. From the reading, 0*05 c.c. 

 was deducted as being necessary to produce the change of tint 

 over the neutral point. For example, 25 c.c. of an extract gave 

 a tint with 015 c.c. of N/TOO soda. Deducting 0*05 c.c, the 

 reading becomes O'l c.c. for the neutral point. With 0-4 c.c, 

 the tint was a pronounced red, and the final deep purple was 

 obtained with 0-9 c.c. The neutral quantity 0*1 c.c, when cal- 

 culated to normal acid per litre, gives 0-04 c.c, which, by Fuller's 

 scale, is represented by -f 0'04. A good, white light is required 

 during the titration to observe the change of tint. The method 

 appears to be correct, for when the extracts were neutralised by 

 the findings of the method, they always gave the highest counts 

 after incubation with the test-bacterium. 



After determining the reaction of the extract, quantities of 

 100th normal lactic acid or bicarbonate of soda were added to 

 10 c.c portions of the extract, and sterile water to bring the 

 volume up to 1 1 cc. One c.c. of a suspension of Bac. jyrodigiosus 

 was added, and this brought the volume up to 12 c.c, upon which 

 the quantities of acid and alkali, which were added, had been 

 based. The bottles were incubated at 22" for 20 hours, when 

 counts were made, and these were calculated in terms of the 

 neutral extract. 



