176 CHEMICAL INFLUENCES. 



considered efficient under any other condition, will destroy microorganisms 

 in pure water. 



The age of the culture and the stage of development will naturally 

 change the resistance of a species materially. The old cultures which 

 are past the culmination of growth will be much more sensitive to any 

 poison unless a spore-producing organism is under test. In this case, 

 we find a greatly increased resistance, similar to the increased resistance 

 of spores against drying and heat. 



THE CLASSIFICATION OF DISINFECTANTS is very difficult as long as we 

 cannot explain completely the process of poisoning. It is impossible to 

 arrange them according to the intensity of action, because the intensity 

 of influence depends not only upon the disinfectant, but also upon the 

 species of organisms. Some yeasts can resist ten times as much alcohol 

 as certain bacteria. Formaldehyde is not nearly as strong an agent with 

 molds as it is with bacteria. The disinfectant concentration of a 

 poisonous substance is not absolute. The simplest method of grouping 

 is by chemical structure and qualities. Of the following natural groups 

 can be distinguished acids (inorganic and organic) , metallic salts, hydro- 

 carbons (aliphatic and cyclic), alcohols (aliphatic and cyclic), aldehydes, 

 anaesthetics, essential oils, oxidizing agents and reducing agents. 



The first three groups, acids, alkalies and salts, are distinguished from 

 the rest as electrolytes; the strength of acids and alkalies (chemically 

 speaking) is measured by the degree of electrolytic dissociation. The 

 disinfectant value follows largely the same law. The strongest acids in 

 the chemical sense are also the strongest disinfectants. There are ex- 

 ceptions, however, where, besides the poisonous effect due to the degree 

 of dissociation, there is a specific effect due to the chemical structure, as 

 in the case of nitrous, salicylic and hydrocyanic acids. The same is true 

 of alkalies. With metallic salts, the action will depend mainly upon the 

 metal in solution, but the electrolytic dissociation is also of importance. 

 NaCl will decrease the dissociation of mercuric chloride (HgCl 2 ) and de- 

 crease also its disinfectant power. Mercuric chloride dissolved in abso- 

 lute alcohol is not dissociated. In this case, it has almost no action upon 

 bacteria. 



Acids are not commonly used as disinfectants, except in the household, 

 but they play a certain role in nature The common fruits contain so 

 much acid that bacteria cannot easily attack them; the decaying of fruit 

 is almost exclusively due to molds which have a preference for acid media; 



