DISINFECTION AND DISINFECTANTS 247 



certain that in most cases the extent and rate of dis- 

 infection to be had from a solution depend on the extent 

 and rate of its penetration into the bacterial cell, and that 

 accordingly an increase of concentration above that of 

 dilute solutions may not be accompanied by a pro- 

 portionate increase of disinfectant power. Adsorption 

 of the disinfectant is the first phase in disinfection, 

 chemical action of the disinfectant on the micro-organism 

 being the second phase. The sum, however, of all the 

 known facts fails to establish any general chemical 

 criterion by which the germicidal efficiency of a substance 

 can be foretold. 



Where salts of an analogous composition are concerned, 

 disinfection is found to be more efficient as the amount of 

 dissociation increases. Paul and Kronig found that solu- 

 tions containing a toxic ion in the same proportion have 

 an identical toxic action . Mercuric chloride and mercuric 

 bromide are almost equally dissociated in solution, and are 

 of equal power as disinfectants in dilutions of 1 gramme 

 molecule in 64 litres. Mercuric cyanide undergoes less 

 dissociation, and is a weaker disinfectant in dilutions of 

 four times the strength. The efficiency of a solution of 

 mercuric chloride is considerably diminished by the 

 addition of sodium chloride, which illustrates the general 

 principle that, when to a solution containing free ions 

 similar ions are added, there is a tendency to re-form the 

 original molecules. 



Water ionises most electrolytes; glycerin and acetic 

 acid are indifferent dissociants; while substances dissolved 

 in chloroform, ether, or benzene ionise but little or not at all. 



Chick found the destruction of bacteria by water 

 between 45 and 55 to be a consistent time process, 

 and to run parallel to the heat coagulation of proteins. 



Madsen, Nyman, and Chick have shown that relatively 

 larger number of organisms are killed when contact with 

 disinfectant commences than after, when the rate of killing 

 gradually falls. They found that if the results be plotted, 

 ordinates showing the numbers of surviving organisms and 

 abscissae the corresponding times, the points lie on a 

 hyperbolic curve. The curve is expressed by the formula 



log - 1 = K, when n-, and 7? 2 are the numbers of 



*2 *1 W 2 



bacteria alive after the times ^ and t 2 respectively, and K 

 is a constant. 



