238 CHEMICAL INFLUENCES 



emulsions. The disinfecting properties of tar come from the cresol 

 contained in it. 



Hydrocarbons are used only for laboratory experiments as very 

 weak antiseptics. The aliphatic bodies, as methane, etc., which con- 

 stitute a large part of coal gas, have very little if any effect upon bac- 

 teria; gas is used occasionally in place of hydrogen for growing anae- 

 robic bacteria. Benzol, xylol, and toluol are antiseptics, if shaken 

 frequently with the hquid to be protected, but they are not reliable 

 as disinfectants. The same is true with the comon anaesthetics, ether 

 and chloroform. The high prices of these agents forbid their general 

 use, but they are sometimes used for laboratory work. 



The essential oils have a Uttle more practical importance. Some of 

 these are the main constituents of mouth washes, especially the oil of 

 peppermint (menthol), of thyme (thymol), and of eucalyptus (eucalyp- 

 tol). Their action is very weak, however. The volatile oils of spices 

 have to be considered in the preserving of fruit, pickles, catsups, and 

 other food products. Though the antiseptic value in general is insigni- 

 ficant, certain microorganisms are sensitive to certain spices. The 

 bacteria of the mesentericus 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 

 eifficient spices, while the disinfectant powder 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 steril- 

 izing rooms. It kills spores of bacteria in a short time in a i : 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 alsd 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- 



