DISINFECTANTS AND DISINFECTION. 165 



6. It is known that the disinfecting power of metallic salts is proportionate 

 to their electric dissociation, that is, the more strongly a salt is dissociated by 

 electrolysis the stronger is its disinfecting power. It follows that anything 

 which interferes with the electrolytical dissociation of germicides weakens 

 the germicidat power. For example, the addition of sodium chloride lowers 

 the germ destroying powers of corrosive sublimate through such interference. 

 This is a matter of great importance in determining the value of antiseptics. 



7. The chemical composition of the material associated with the germs 

 to be destroyed has a marked influence upon the action of the germicides. 

 Thus germicides give different results when acting upon the same organism in 

 water, in beef broth, in salt solutions, in and upon tissues, etc. For this 

 reason the value of germicides in actual practice cannot be based exactly 

 upon the laboratory results. 



8. Not only do different species of disease germs differ in resistance to 

 germicides, but the different strains of the same species react differently with 

 the same germicide. Certain organisms appear to have an elective affinity 

 for certain chemicals, as for example, the malaria germ for quinine, and the 

 syphilis germ for mercury salts. 



Disinfectants destroy or kill germs in different ways. In some cases 

 the death of the organism is due to oxidation as when ozone, hydrogen 

 peroxide and sulphites are used, or death may be due to interference with 

 nutrition, but more generally it is due to the coagulation of albumen and 

 abstraction of water from the cell-plasm, as in the use of dry heat, phenol, 

 alcohol, tannic acid and metallic salts. As already explained in another 

 chapter, lysins act by actually disintegrating the bacterial cells. 



The action of disinfectants depends upon a great variety of conditions, 

 entirely too numerous and too complicated to be fully treated in a text-book, 

 nor is it necessary to enter into lengthy discussions and explanations. In 

 time many of the points still in dispute will be definitely worked out, in the 

 laboratory. 



As a rule germicides are most active when dissolved in water, though 

 some authorities declare that bichoride of mercury, phenol, thymol and lysol 

 are more active when dissolved in 50 per cent, alcohol. The Activity N of 

 phenol as a germicide is greatly increased by the addition of hydrochloric 

 acid, whereas lime reduces its potency. Solutions of germicides in oils 

 are inert because oil does not penetrate the bacterial cell; however, the oil 

 itself may be fatal to bacterial life, in which case the added germicide is un- 

 necessary. Chemical germicides do, however, increase the potency of the 

 volatile coal-tar products as gasoline, benzine and xylol, provided the 

 germicides are soluble in these substances. 



The following are the more important disinfectants given approximately 

 in the order of their usefulness and potency. 



