184 CONTRIBUTIONS TO UUR KNOWLEDGE OF SOIL-FERTILITY, xvi., 



Experiment xxvii. 



The distilled water test shows that the bacteria grow best in 

 a neutral solution, and the tap- water test that neutrality is ob- 

 tained when approximately +0-1'' of acid has been added. The 

 curves of these numbers are steep on both sides of the approxi- 

 mately neutral line, indicating that a slight difference in the 

 reaction of an extract will make a great difference in the growth- 

 numbers. As it is impossible to obtain a neutral reaction in the 

 cultivated extracts, it would be necessary to neutralise them 

 before dilution in such tests as have been made. It is not clear 

 how this could be done, for, in the solution under examination, 

 the bacteria showed 0'1° of alkalinity, methyl-orange showed 0-'2°, 

 phenolphthalein by direct titration after boiling, 003°, and by 

 indirect or back titration, 0*14°. 



The experiment would be incomplete without the inclusion of 

 others showing the influence of diluting distilled and tap-water 

 infusions with distilled water, as is customary. 



