INHIBITION OF GROWTH 295 



monly used instead of the liquid, because it is more easily handled and is 

 quite inoffensive in its solid form, while the formaldehyde solution has a 

 very penetrating odor and is exceedingly harmful to the mucous mem- 

 brane of the respiratory organs. 



Of the oxidizing agents, oxygen itself has already been mentioned. 

 Though it is able to destroy certain anaerobic bacteria, it cannot be 

 called a disinfectant. For this purpose, oxygen must be activated ; such 

 oxygen can be obtained in the form of ozone (O 3 ). It is formed in air 

 under the influence of electric discharges and can be produced at a price 

 low enough to allow its application for use in the sterilization of water. 

 It has also been recommended for preservation of milk. 



Hydrogen peroxide (H 2 O 2 ) resembles ozone in its chemical reactions ; 

 it changes readily to H 2 O + O, and this oxygen atom in the nascent 

 state is quite effective as an oxidizing agent. For an antiseptic, it must 

 be used in at least a i per cent solution, and for an absolutely reliable dis- 

 infectant a still higher concentration is required. It loses its disinfect- 

 ing property easily because it is decomposed readily by the peroxidases 

 of tissues and organic liquids as blood, milk, and pus. It is used in the 

 preservation of milk. Hydrogen peroxide is slowly decomposed by the 

 katalase of milk thus disappearing completely. 



Chlorine in its gaseous form is not used as a disinfectant, though its 

 germicidal power is quite strong. The so-called " chloride of lime," 

 manufactured by absorbing chlorine in slaked lime, gives in water 

 hypochlorite and free chlorine; these substances are good germicides 

 and chloride of lime is used in the disinfection of privy vaults, and other 

 places in which it may be employed without injury. Hypochlorite is 

 now used with great success for rendering safe drinking water and 

 sewage; it has also become the basis of some commercial dis- 

 infectants. 



Potassium permanganate is only incidentally used as a disinfectant. 

 Its chemical qualities prevent an ordinary use. 



Sulphurous acid, or sulphur dioxide (SO 2 ) was for a long time a 

 standard disinfectant and is still used occasionally for fumigating rooms, 

 stables, barns and out-buildings though it is substituted more and more 

 by formaldehyde which can be applied almost as easily. The burning 

 of sulphur is an extremely simple process, but it requires a moist air to 

 disinfect properly, and under these circumstances it will attack metal, 

 dyes of clothing and even the fiber itself. 



