SPECIAL EXAMINATIONS 377 



DISINFECTANTS. 



A disinfectant or germicide is a substance which destroys 

 the microbic causes of disease. The same substance 

 in a weaker strength may act as an antiseptic, that is an 

 agent which restrains or checks the growth of bacteria 

 without destroying them. A deodorant is a substance 

 which destroys or masks the offensive effluvia or vapours 

 resulting from bacterial growth, and may or may not 

 have an action on the bacteria themselves. The best 

 example of an agent capable of all these functions is 

 potassium permanganate, which in strengths of 5 per cent 

 and over is a disinfectant, under 5 per cent is an anti- 

 septic, and in all strengths is a deodorant. (Solubility, 

 1-18 of water ; 1-3 boiling.) The mode of action of 

 disinfectants varies. Some, like the strong acids, char or 

 carbonize the bacterial body by oxidizing the other 

 elements present. Others coagulate the bacterial proto- 

 plasm ; while some, diffusing through the cell wall, exert 

 a poisonous action on the protoplasm. Antiseptics like 

 sugar, probably act by osmotic pressure through the cell 

 wall, causing a flow of water out of the cell, thus drying 

 up the protoplasm and rendering the bacterium inert for 

 the time being. 



The chief disinfectants are the metallic salts, acids 

 bases, the halogen elements, oxidizing agents, alcohols 

 phenols, aldehydes, and the essential oils. The salts, 

 acids, and bases act best in watery solution as against 

 alcoholic solutions. Two explanations of this are offered : 



(1) That the salts, etc., in water dissociate into their ions, 

 and the latter are more active in producing chemical 

 change ; in alcohols, dissociation does not take place. 



(2) That the alcohol hinders the action of the disinfectant 

 by hardening the bacterial cell wall. In favour of the 

 second reason it has been noted that while absolute 

 alcohol is useless as a germicide, added to aqueous mercuric 

 chloride solution in the proportion of 25 per cent it increases 

 the efficiency of the disinfectant. The addition of NaCl, 

 on the other hand, diminishes the efficiency, it is believed 

 by reducing the number of free ions. 



The halogens (Cl, Br, I) are efficient in the order given. 

 Chloride of lime liberates free Cl when treated with an 



