DISINFECTANTS 373 



Disinfectants. — Certain substances prevent the growth of 

 micro-organisms. If they do so in such a manner as to 

 permit the organisms to grow again when removed from 

 the restraining influence and sown on a suitable medium, 

 they are called antiseptics ; if they do not, and if they also 

 remove their capacity to infect a susceptible living animal, 

 they are called germicides, or disinfectants. In most cases, 

 but not in all, an antiseptic is merely a germicide, or dis- 

 infectant, in a dilute condition. In the following pages we 

 shall allude to both classes of substances as disinfectants. 



Among the large number of chemical compounds that 

 have well-marked germicidal or antiseptic powers, depend- 

 ing upon the strength and conditions under which they 

 are used, are the following : The free acids and alkalies, 

 chlorine, bromine, iodine, ozone, hydroxyl, sulphurous acid, 

 hypochlorites, sulphites, the salts of mercury, zinc, and 

 copper, boric acid, fluorides, the manganates and perman- 

 ganates, carbolic and salicylic acids, chloroform, iodoform, 

 formic aldehyde (formalin), chinosol, etc. In addition to 

 these, many essential oils, and a vast number of organic 

 compounds, particularly of the aromatic series, have been 

 credited by various investigators as having more or less 

 marked germicidal or antiseptic powers. 



The Bacteriological Testing of Disinfectants. — The ex- 

 amination of a substance for disinfectant capacity is an 

 extremely complex matter. It has not always been 

 recognised as such ; and no department of bacteriological 

 inquiry is more encumbered with inconclusive researches 

 than this investigation of disinfectants. The conditions 

 which determine the death of an organism in the presence 

 of foreign substances, or its cessation of growth, vary not 

 only for different species, but also for different races of 

 apparently the same species, according to their previous 

 history, and for a single race at various ages. They are 



