DISINFECTANTS AND DISINFECTION 325 



tent. Agitating and churning contaminated liquids checks bacterial 

 development somewhat. The active circulation of contaminated air 

 reduces the number of bacteria present. Agitation is, however, not. a, 

 satisfactory means of sterilization and disinfection. 



/. Sedimentation and Filtration. Sedimentation in sewage waters and 

 other contaminated liquids, combined with nitration, is a very effective 

 means of purification. Precipitation and filtration, aided by chemicals as 

 alum, iron sulphate, and other coagulants, are much employed in the puri- 

 fication of water supplies. 



g. Free Circulation. Free circulation of air and water are most favor- 

 able to sanitation because of the checking influence upon bacterial activity 

 and also because of the disseminating and diluting effects upon the organ- 

 isms which may be present. Circulation is strictly speaking a form of 

 cleansing. 



Purification of flowing water, as rivers and small streams, is effected 

 very largely by oxidation and dilution. The agitated water takes up 

 oxygen by absorption which combines with the organic particles suspended 

 in the water rendering it unsuitable as food for bacteria. As the water 

 flows along, the bacteria are scattered more and more. Sedimentation 

 is also an important factor in the destruction of bacteria. Gradually 

 the bacteria settle to the bottom of the stream where they are brought 

 in competition with other bacteria, protozoa, algae, perhaps hyphal 

 fungi, etc., which tend to check and even entirely inhibit their further 

 development. 



h. Light. Sunlight has most marked germicidal powers, due in part, 

 to the drying effects produced and in part to the actinic or chemically 

 active rays of the sun's light. Numerous investigators have demonstrated 

 the germ-destroying effects of the blue and violet-rays and the ultra violet 

 end of the solar spectrum. Bacteria cannot survive in sunlight. Electric 

 light is said to have the same effect upon bacterial h'fe as sunlight. The 

 J\T-rays destroy bacteria, likewise does radium, and these agents have been 

 extensively tested in the treatment of skin diseases and superficial tubercu- 

 losis as lupus, and in cancer, but without satisfactory or conclusive results. 



i. Electricity. The electrical current in itself appears to be without 

 germicidal powers, but electricity is used to precipitate smelter fumes, 

 and organic impurities in water, as already stated. Electricity is used 

 to stimulate seed germination and it may be possible to utilize electrical 

 discharges or currents in the treatment of communicable diseases. 



2. Chemical Disinfectants 



Chemical disinfectants may be divided into gaseous (or vaporous) and 

 liquid (solutions). The liquid disinfectants are superior to the gaseous 

 disinfectants because direct contact with the articles to be disinfected can 



