INHIBITION OF GROWTH. 179 



The essential oils have a little more practical importance. Some of these 

 are the main constituents of mouth washes, especially the oil of pepper- 

 mint (menthol) , of thyme (thymol) , and of eucalyptus (eucalyptol) . Their 

 action is very weak, however. The volatile oils of spices have to be con- 

 sidered in the preserving of fruit, pickles, catsups, and other food products. 

 Though the antiseptic value in general is insignificant, certain micro- 

 organisms are sensitive to certain spices. The bacteria of the mesen- 

 tericus group are said to be suppressed entirely by quite small quantities 

 of garlic, while others, like the lactic bacteria, are not affected at all. 

 Cloves, cinnamon and alspice are the most efficient spices, while the 

 disinfectant power of black and white pepper, and mustard is very small. 



The most important disinfectant has not been mentioned, because 

 it does not belong to any of the above groups. This is formaldehyde. 

 Formaldehyde (HCOH) is a gas, soluble in water to the amount of 40 

 per cent at room temperature; it does not attack metal, clothing, wood- 

 work, and is, therefore, preferable to many other disinfectants for steriliz- 

 ing rooms. It kills spores of bacteria in a short time in a 1:1000 di- 

 lution. Its greatest importance lies, however, in its gaseous nature, 

 because it can be applied to rooms and buildings by simply evaporating it. 

 The saturated 40 per cent solution can be evaporated directly or by 

 generating steam which passes through the formaldehyde solution; this 

 latter method has the advantage of saturating the air with moisture, 

 which increases the power of the formaldehyde gas. Formaldehyde 

 can also be obtained in a dry form; it polymerizes to a white crystalline 

 substance, paraformaldehyde ((HCOH) 3 ) which can be changed back to 

 formaldehyde gas by gentle heating. This paraformaldehyde is com- 

 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; 



