1. In the growth and death of the pneumococcus in fluid media containing 1 per cent glucose the production of acid is the most important bactericidal factor.
2. 1 per cent glucose bouillon cultures of the pneumococcus allowed to grow and die out usually reach a final acidity of a pH of about 5.1.
3. At a hydrogen ion concentration of about 5.1 or higher, the pneumococcus does not survive longer than a few hours.
4. In hydrogen ion concentrations of about 6.8 to 7.4 the pneumococcus may live for at least many days.
5. In the intervening hydrogen ion concentrations, between 6.8 and 5.1, the pneumococcus is usually killed with a rapidity which bears a direct relation to the hydrogen ion concentration; i.e., the greater the acidity the more rapid is the death.
6. Cloudy suspensions of washed pneumococci in hydrogen ion concentrations varying from 8.0 to 4.0 show, after incubation, dissolution of organisms in lower hydrogen ion concentrations than about 5.0. This dissolution is most marked at about 5.0 to 6.0. Some dissolution also takes place toward the more alkaline end of the scale. No dissolution occurs at the most acid end of the scale.