74 BACTERIOLOGY. 



course of bacterial growth, development of the 

 germs ceases when a certain definite proportion 

 of the acid in solution is reached. This propor- 

 tion has been accurately determined by Hueppe 

 in the case of lactic acid, and a similar inhibiting 

 point has been shown to exist in the case of the 

 alcohol produced by yeast. If the lactic acid is 

 neutralized by the addition of carbonate of lime, 

 fermentation starts up again. Prom these facts 

 was deduced a principle of immediate practical 

 importance, viz., that quick-lime and milk of 

 lime are disinfecting agents only so long as 

 sufficient calcium hydroxide (Ca (OH) 2 ), is pre- 

 sent. If this compound is converted into car- 

 bonate of lime by the carbon dioxide of the 

 atmosphere or in solution, then the substance 

 is a disinfecting agent no longer, but, on the 

 contrary, a favorable medium for bacterial 

 vegetation since a feebly alkaline reaction of 

 the nutrient medium generally favors bacterial 

 growth. In fruit preserving, again, it is of 

 interest to know that the vegetable acids that 

 are present check bacterial growth but permit 

 the vegetation of moulds. 



The hydrochloric acid contained in the gas- 

 tric juice constitutes an important means of 

 defence against bacterial invasion. This sort 

 of safeguard is not, however, developed to an 

 equal extent in different individuals and 



