C.-E. A. WINSLOW 75 



for example, by Schaffer andTilley [1927] for the various members of the alcohol and 

 phenol group) is a fascinating one. 



Chick (1908) showed that within certain limits the efficiency of a chemical disin- 

 fectant bears a logarithmic relation to its concentration, the expression 



log 7^ 



In to L'oto 



remaining constant in value when to and /„ are the time periods necessary for disinfec- 

 tion corresponding to concentrations Co and C„. Furthermore, it is important to note 

 that disinfectants not only vary in their efficiency at a given concentration but also 

 vary in the degree to which their toxicity increases with increasing concentration. 

 The ordinary carbolic acid coefficient (representing the ratio of the concentration of a 

 given disinfectant to the concentration of carbolic acid which will sterilize in a given 

 time) therefore gives a very incomplete idea of the true relationships. Thus Chick 

 (1908) found that mercuric chloride may have a carbolic acid coefficient of 13 at one 

 concentration and of 550 at another. 



Phelps (191 1 ) therefore suggests that values of K (K and K') should be deter- 

 mined at two different concentrations (C and C). If we express the K of our previous 

 formula (p. 72) by KC" (to allow for this concentration factor) we get 



log ^=KCH 

 and 



log ^=K'C'H 

 Therefore 



n=Iog-^ '^^^S'C 



From the two determinations of K we can then compute n or the concentration 

 coefficient of the particular disinfectant studied. This figure n shows by what power 

 of 2 the efficiency is increased if the concentration be doubled. For anthrax spores in 

 mercuric chloride at 20° C. it is 1.08. 



The reaction of the medium is a special case of chemical toxicity which has been 

 studied with particular care. Such a phenomenon occurs in nature in streams re- 

 ceiving acid wastes, and the disinfectant action of carbon dioxide is due to the same 

 cause (Koser and Skinner, 1922). As pointed out above, carbon dioxide, aside from 

 its effect upon reaction, is beneficial and indeed essential to bacterial life. 



Winslow and Lochridge (1906) reported twenty years ago that the toxic effect of 

 mineral acids was in large measure due to dissociated hydrogen. Cohen (1922) has 

 given us one of the most careful recent studies of this problem, and we may cite one of 

 his summary tables (Table VI) indicating the effect of hydrogen-ion concentration 

 upon the death-rate of Bad. typhosum. 



The effect of cations other than hydrogen upon bacterial growth and death has 

 been studied by numerous observers (see review by Falk, 1923). Those who have 

 worked on the increase of bacterial populations in favorable media, from Richet 



