DISINFECTANTS 111 



most often used in classification are according to either composition 

 or mode of action. The simplest method is by chemical structures 

 and qualities under which are distinguished the following natural 

 groups: acids, alkalies, metallic salts, hydrocarbons, alcohols, 

 aldehyds, anesthetics, essential oils, and oxidizing and reducing 

 agents. The first three acids, alkalies, and salts are distinguished 

 from the rest by being electrolytes. The strength of acids and alka- 

 lies is dependent upon the hydrogen or hydroxyl ion concentration 

 with the metallic salt; the action is dependent upon the nature of 

 the metallic ion and the degree of electrolytic dissociation. 



Rosenau classified disinfectants according to mode of action as 

 follows : (1) Those compounds which destroy by oxidation, as ozone, 

 chlorinated lime, potassium permanganate, and the halogens. 

 (2) The destruction by ionic poison with coagulation, as the metallic 

 salts, mercury, and lead salts. (3) Destruction by coagulation and 

 poisoning not ionic in character, as carbolic acid and its derivatives. 

 (4) Destruction by emulsoid action, that is, through Brownian 

 movement and adsorption; soap solutions and creolin. 



Laws Governing the Action of Disinfectants. These have been 

 mainly worked out by Chick who found that disinfection is an 

 orderly time-process, which may be considered analogous with a 

 chemical reaction, viz., a reaction between the bacterium on the 

 one hand and the disinfectant on the other. In the ideal case disin- 

 fection proceeds in accordance with some rule analogous to the mass 

 law, so that if the disinfectant is present in large excess, disinfection 

 rate at any moment is proportional to the concentration of bacteria 



( - -, = Kn, where n is the concentration of bacteria at the time t, 



and K is a constant, depending on the temperature concentration 

 of disinfectant, etc.). 



The velocity of disinfection increases with rise in temperature in 

 an orderly manner according to the well-known equation of Arrhe- 

 nius. Some idea of the magnitude of the effect of temperature may 

 be gained from the fact that with metallic salts the mean velocity 

 of disinfection increases two- to four-fold for a rise in temperature 

 of 10 C., whereas with phenol it was as high as eight-fold, using 

 B. paratyphosus as the test organism in each case. Hence, the use 

 of a disinfectant at a comparatively high temperature, other things 

 being equal, is more effective than its use at a low temperature. In 

 reality, a solution which at one temperature is only an antiseptic 

 may become a disinfectant by a small increase in temperature. 



The efficiency of a disinfectant varies with the moisture. A dry 

 poison has but slight action on microorganisms. For this reason, 

 dry formaldehyde gas is practically without effect. In a similar 

 manner absolute alcohol has not nearly the same germicidal power 

 as has 50 to 70 per cent, alcohol. This is probably due to the 

 absolute alcohol coagulating the outer membrane of the organism 



