INFLUENCE OF MECHANICAL EFFECTS. 169 



ments. It was mentioned in that connection that Bact. butyricum cannot 

 tolerate more than 0.65 per cent of the total oxygen^o.2 atm.); content 

 in air in other words, an oxygen pressure higher than 0.0013 at- 

 mospheres will kill the organism. The maximum pressure for B. 

 prodigiosus was found to be about 5.4 to 6.3 atmospheres. Very few 

 experiments have been made with other gases. Carbon dioxide at a 

 pressure of 50 atmospheres retards the growth of bacteria in water and 

 will sterilize it in twenty-four hours. Suspensions of pure cultures of 

 B. lyphosus and Msp. comma are killed by 50 atmospheres carbon dioxide 

 pressure in three hours. Milk cannot be sterilized by this pressure but 

 bacteria do not multiply. Carbonated milk has been recommended as 

 a refreshing drink by several investigators. The ordinary market milk 

 will keep about two days longer under the pressure of 10 atmospheres 

 (150 pounds) than without pressure. If pasteurized it is said to keep 

 for a week. 



GRAVITY. Gravity would have a great influence upon the growth of 

 microorganisms in liquids if their specific gravity were much greater than 

 that of water. This does not seem to be the case however. It has been 

 estimated by accurate weighing to vary between 1.038 and 1.065. Very 

 much higher results (1.3 to 1.5) have been obtained by centrifuging bac- 

 teria in salt solutions of varying specific gravity, but these data are not exact 

 since the salt solution will diffuse into the cells and thus increase their 

 weight. The specific gravity being very nearly that of the culture medium, 

 it is plainly seen that gravity has but little influence. The microorganisms 

 will live suspended in the liquid and sediment out very slowly. The 

 slightest current in the liquid will carry them around and distribute them 

 through the medium. The motility is of minor importance; the actual 

 distance covered by motile bacteria has been measured, and under the 

 most careful exclusion of currents in the liquid has been found to be about 

 a millimeter in a minute for B. subtilis. This is very slow compared with 

 the speed of the circulating water moved by changes of temperature or 

 other incidental agents. 



Yeast cells and other gas producers use the carbon dioxide as a vehicle. 

 The gas bubbling up in the fermenting liquid keeps it constantly in motion 

 and moves the yeast cells against gravity toward the surface where the gas 

 escapes and lets the cells fall back to the bottom. 



The production of scums and pellicles on the surface by organisms, 

 which are heavier than the liquid they float on, is often accomplished by 



