240 MANUAL OF BACTERIOLOGY. 



phenol or to formaldehyde lessens the germicidal property of 

 these germicides. The addition of even 5 per cent, of alcohol 

 shows this effect. On the other hand, the addition of alcohol 

 to aqueous solutions of corrosive sublimate or silver nitrate 

 increases the germicidal power of these germicides. The 

 addition of glycerin to solutions of germicides acts irregularly, 

 increasing the power in some of them, decreasing the power in 

 others. 



Researches in physical chemistry have shown that the disin- 

 fecting power of metalic salts is in proportion to their electro- 

 lytic dissociation, the more strongly dissociated a salt is by 

 electrolysis, the stronger its disinfecting power. 



Scheurlen and Spiro have shown this with iron salts. From 

 their results it appears that only those salts of iron act as 

 germicides in which the iron is present as kation. On the 

 other hand, all disinfectants do not act in this way. Phenol 

 and its derivatives do not act through the ions. In this case 

 it is precisely those molecules which are not dissociated which 

 act as germicides. Sodium phenolate is much more strongly 

 dissociated than phenol itself, and yet phenol is much more 

 strongly germicidal than the salt. But with the metalic salts 

 anything which interferes with their dissociation weakens 

 their disinfecting power. The addition of sodium chloride 

 interferes with the dissociation of corrosive sublimate and 

 weakens its germicidal power. This is shown more strongly 

 in concentrated than in dilute solutions. In the dilute solu- 

 tions of corrosive sublimate ordinarily employed in practice the 

 addition of the salt does not weaken the germicidal power 

 very markedly. 



Temperature has a marked effect upon the action qf ger- 

 micidal agents. The higher the temperature, the more potent 

 the germical action, but just the reverse is true in regard to 

 the inhibitory action of germicides. The inhibitory action is 

 weakest at the optimum temperature for the growth of the bac- 



