MODE OF ACTION OF SALTS 127 



centration of the same salt. How far this explanation is of general applicability, it is as 

 yet impossible to say. 



The interaction of various salts is of considerable importance in disinfection. 

 The germicidal action of any one salt may be increased or diminished by the 

 addition of any other. For example, the addition of NaCl is said to lower the 

 toxicity of HgClj, but to increase the toxicity of mercuric nitrate, sulphate, or 

 acetate. In estimating, therefore, the toxicity of a salt, the saline content of the 

 solution in which it is acting must be defined. Norton and Hsu (1916) showed 

 that salts were able to modify the germicidal power of acids. When ammonium 

 formate was added to formic acid, the H-ion concentration of the solution decreased 

 as a result of an increase in the concentration of undissociated acid molecules, 

 and its disinfectant power was lowered ; when sodium nitrate and sodium chloride 

 were added in very small quantities to formic acid, the degree of dissociation of the 

 acid was hardly affected, but its disinfectant power was considerably increased. They 

 conclude therefore that the addition to an acid of a salt containing an anion common 

 to this acid diminishes its disinfectant power ; the addition of a salt which does 

 not have any appreciable effect on the dissociation of the acid greatly increases its 

 disinfectant power. 



Not only do salts assist or antagonize the action of each other ; they have a similar 

 effect on disinfectants of quite different chemical constitution. Scheurlen (1895) 

 showed, for instance, that the addition of sodium chloride in a concentration of 24 

 per cent, to a solution of phenol increased its disinfectant power. Beckman (1896) 

 confirmed this, and found that with staphylococci the addition of even 1 per cent. 

 NaCl to 1 per cent, phenol apparently increased its activity. With anthrax spores, 

 the addition of 6 per cent. NaCl to 1 per cent, phenol had no effect ; 12 per cent. 

 NaCl increased its activity slightly, and 24 per cent. NaCl increased its activity 

 very greatly. Thus 1 per cent, phenol alone failed to kill a suspension of 24,800,000 

 spores in 8 days ; 1 per cent, phenol + 24 per cent. NaCl killed them completely 

 iu between 5 and 24 hours. Eomer (1898) confirmed the work of Beckman, 

 showing that the greater the amount of salt added^ the greater was the increase 

 in disinfectant power. As a rule, the more toxic a salt is in itself, the more does 

 it supplement the action of the disinfectant (Eisenberg and Okolska 1913). 



Mode of Action of Salts.^ — In endeavouring to explain the action of salts on 

 bacteria, we must remember that we are dealing with a complex problem of which 

 there is no simple solution. Many factors are concerned, and the most we can do 

 here is to discuss the most important in turn. 



(1) The Osmotic Effect. — It is doubtful whether salts, except in high concen- 

 trations, exert any influence on bacteria by virtue of their osmotic pressure. 

 Bacteria differ in this way from practically all other living cells. Thus Fischer 

 (1900) observed that B. suhtilis grew well in an infusion containing 9 per cent. 

 NaCl, 11 per cent. KCl, or 10 per cent. KNO3 ; and Knaysi (19306) found that 

 to demonstrate plasmolysis in this organism a 25 per cent, solution of NaCl was 

 required. Though salts have Little direct osmotic action on bacteria, they may 

 exert an indirect action by causing a dehydration of the proteins on which the 

 organisms are growing. It is this dehydrating action of salts which is relied on in 

 many processes of food preservation. 



(2) Oxidation. — Salts and certain allied bodies that contain a high proportion 

 of oxygen, or that are able to liberate oxygen from other compounds, have long 



