88 BIOLOGY AND TECHNIQUE 



complex anion Ag (CN) 2 , this latter further dissociating to a very 

 slight degree only. The same writers conclude that the bactericidal 

 action of mercuric chloride and of halogen combinations with metals 

 is directly proportionate to the degree of dissociation. This con- 

 sideration, moreover, explains why aqueous solutions of such sub- 

 stances are more active than are solutions in the alcohols or in 

 ether, since it is well known that metallic salts are ionized in these 

 substances to a much slighter degree than they are in water. 35 



On the other hand, the addition of moderate quantities of ethyl 

 and methyl alcohol or acetones to aqueous solutions of silver nitrate 

 or mercuric chloride, definitely increases the disinfecting action of 

 such solutions. In the case of mercuric chloride, Kronig and Paul 

 obtained the most powerful effects in solutions to which alcohol had 

 been added in a concentration of 25 per cent. For this empirical 

 fact a satisfactory explanation has not yet been found. Kronig 

 and Paul suggest that low percentages of alcohol may facilitate 

 the penetration of the disinfectant through the cell membrane and 

 thus increase its efficiency, while high percentages of alcohol have 

 the opposite effect, by decreasing the degree of dissociation. In 

 this connection it has been suggested, however, that absolute and 

 strong alcohols possibly act as desiccating agents, thus actually 

 rendering the bacteria dry and less susceptible to deleterious 

 chemical influences. 



In the case of acids and bases the same authors have determined 

 that the powers of disinfection of these substances are again directly 

 proportionate to the degree of their dissociation: that is, to the 

 concentration of the hydrogen or hydroxyl ions, respectively. The 

 hydrogen ions are more powerfully active than the hydroxyl ions 

 in equal concentration; acids, therefore, are more efficient disin- 

 fectants than bases. 



A fact which appears to strengthen the opinion as to the relation- 

 ship between bactericidal powers and dissociation, is that brought 

 forward by Scheuerlen and Spiro, that the addition of NaCl to 

 bichloride of mercury solutions reduces the disinfecting power of 



* 5 Water is the strongest 'dissociant known. Methyl alcohol has about one-half 

 to two-thirds the dissociating power of water (ZelinsTcy, Zeit. f. physiol. Chemie, 

 xx, 1896). Ethyl alcohol allows dissociation much less than methyl alcohol; 

 ammonia allows dissociation to about one-third to one-fourth the extent of water. 

 See Jones, "Elements of Physical Chemistry," p. 371. Macmillan, New York, 

 1902. 



