THEORY OF DISINFECTION 629 



are almost inactive. The efficiency of mercuric chloride 

 is markedly lessened by the presence of sodium chloride 

 or other chlorides. Of the oxidising agents, nitric, chromic, 

 chloric, and permanganic acids act in the order stated ; 

 chlorine has the most powerful action of the halogens. 

 Phenol acts better in a 5 per cent, solution than in higher 

 concentrations, and the efficiency is increased by the 

 addition of sodium chloride, but diminished by the presence 

 of alcohol, and under the most favourable conditions it is 

 not such a powerful germicide as mercuric chloride. Mer- 

 curic chloride dissolved in absolute alcohol has little or no 

 efficiency, and the addition of sodium chloride reduces its 

 activity. Organisms in masses are less readily acted upon 

 by antiseptics than when they are isolated. 



The efficiency of a germicidal salt in solution seems to 

 vary with its dissociation. It is believed that the molecules 

 of a salt in solution are more or less dissociated into con- 

 stituent electrified atoms or " ions," and the greater the 

 dissociation the more active will the substance be as a 

 germicide. Taking mercuric chloride, bromide and cyanide, 

 it is found that the ionisation of the chloride is greater 

 than that of the bromide, and this is more ionised than 

 the cyanide, and the following results show that the 

 germicidal power of the three is in this order : 1 



Number of 

 colonies which developed. 



After After 



Solution. 20 minutes' 85 minutes' 



treatment. treatment. 



1 mole HgCl 2 in 64 litres . 7 



1 HgBr 2 . 34 



1 Hg(CN) 2 in 16 litres 8 33 



Since the amount of this dissociation may be greatly 

 influenced by the presence of other substances, much 



1 Findlay, Physical Chemistry, 1905. 



