REACTION VELOCITY 



139 



It will be seen from Table 15 that k has a mean value of 044 ; from Fig. 21 

 that the velocity of the reaction becomes slower and slower, till it is almost neg- 

 ligible (in theory the reaction never reaches completion), and from Fig. 22 that the 

 logarithms of the numbers of sur- 

 viving organisms plotted against tn 30 

 time in hours fall along a descend- 

 ing straight line. 



In the case of vegetative bac- 

 teria, she (Chick 1908, 1910) found 

 that though the disinfection of 

 some organisms such as Sahn. 

 typhi and Bact. coli conformed 

 to the unimolecular reaction 

 formula, with others there was a 

 slight departure from it. Thus 

 with Staphylococcus aureus exposed 

 to 0-6 per cent, phenol at 20° C, 

 there was invariably a lag period, 

 lasting about 4 minutes before the 

 rate became strictly proportional 

 to the number of bacteria (Fig. 23). 

 Paratyphoid bacilli behaved in the 

 opposite way. Instead of there 

 being a lag phase, there was a preliminary rush during which the rate of dis- 

 infection proceeded faster than it should have done according to the equation 



(Fig. 24). 



Fig 



Disinfection of Staph, aureus with 0-6 per cent, 

 phenol at 20° C. The numbers of organisms 

 are expressed logarithmically. 

 (After Chick.) 



20,000 



/o 15,000 





5.000- 



JO 20 30 40 50 60 70 

 Time, in Minutes 



Fig. 24. 

 Disinfection of a 24-hours' culture of para- 

 typhoid bacilli with 0-6 per cent, phenol at 20° C. 

 (After Chick.) 



mathematical evidence in favour of this view. 



It does not, of course, follow from such results as these that a unimolecular 

 reaction, in the chemical sense, is actually taking place ; the recorded observations 



Chick confirmed her work on dis- 

 infection by phenol by showing (1) 

 that the death of Bact. coli under 

 the influence of a bactericidal serum 

 conforms to the unimolecular reaction 

 law (Chick 1912) ; and (2) that the 

 same law holds in the process of dis- 

 infection by hot water (Chick 1910) ; 

 she pointed out the close parallel that 

 exists between disinfection by hot 

 water and the heat coagulation of 

 proteins. Paul, Birstein and Reusz 

 (1910a) found that the killing of 

 Staphylococcus aureus by drying pro- 

 ceeds in accordance with the law of 

 a unimolecular reaction. The results 

 obtained by Clark and Gage (1903) in 

 the study of disinfection by sunlight 

 may also be interpreted in the same 

 Robertson (1914) has adduced 



