76 



THE RISE AND FALL OF BACTERIAL POPULATIONS 



(1892) to Hotchkiss (1923), have found that practically all cations may show either a 

 stimulating or an inhibiting influence, depending on their concentration, and observers 



TABLE VI 



Average Velocity Constants for the Death of Bad. lyphosnm 

 AT Different pH Values, 20° C. 



PH 3-8 S-o 5.4 6.4 7.1 7.6 8.7 9.5 



K 1055 00134 o.oiio 0.0138 0.0437 o. HOC 0.2134 0.2855 



Relative iir*95. 5 1.2 i.o 1.5 4.8 10. o 22.4 31.4 



* Taking K at pH 5.4 as unity. 



like Winslow and Falk (1923) who have studied the death-rate of bacteria in unfavor- 

 able media find that in the same way small amounts of cations favor survival while 



Kl 

 +1.5 



+1.0 



♦0.5 



-0.5 



0.001 o.oi o.f 1.0 



MOLAR CONCENTRATION OF SALT 



1.0 



Fig. 4. — Relation between viability of Bad. coli and salt concentration (Falk and Winslow, 

 1926). 



large amounts increase the mortality. Falk and Winslow (1926) pointed out the sig- 

 nificant fact that if values of K for the effect of NaCl and CaCl2 upon Bad. coli in 

 water be plotted against concentration one obtains a reasonably smooth curve for 

 both salts, passing from positive values (indicating toxicity) to negative values (indi- 

 cating a preservative effect) (see Fig. 4). It will obviously be difficult to harmonize 

 such a phenomenon with any simple chemical assumptions. 



It is also important to remember that the net effect of various chemical and physi- 

 cal factors, acting simultaneously, may be an exceedingly complex one. Thus Chick 

 (1910) showed that very minute excesses of acid or alkali might enormously accel- 

 erate the rate of disinfection by hot water; and the same phenomenon is of great prac- 

 tical importance in connection with heat sterilization. On the other hand, a lethal 

 factor may equally well be neutralized by a favorable one. Sometimes the effect is 

 relatively simple, as in the reduction of the disinfectant power of mercuric chloride 

 due to the presence of organic matter (Chick and Martin, 1908), a fact which makes 



