632 A MANUAL OF BACTERIOLOGY 



action of chemical disinfectants, particularly those acting 

 by oxidation, considerably reducing their efficiency. 



Requirements for an efficient disinfectant. The conditions 

 which should be satisfied by an efficient disinfectant for 

 general use are simple, but not easy to obtain. Because 

 a disinfectant effect depends on the strength of the solution, 

 the substance should have an approximately definite 

 efficiency for particular organisms in given conditions, 

 and for the same reason it should be permanently homo- 

 geneous. In practice disinfectants must be used with 

 water or in an aqueous solvent ; it should, therefore, yield 

 a stable solution or uniform emulsion in all proportions. 

 Because bacteria as presented for practical disinfection 

 usually have some organic coating, it should be stable in 

 the presence of organic matter ; and as this coating is 

 often of a greasy character, it should, especially if intended 

 for use on dirty or greasy surfaces, have high solvent power 

 for grease. For use when heat can also be applied, whereby 

 its activity is enhanced, unless it breaks up, it should be 

 stable at all reasonable temperatures. These conditions 

 may be considered to be indispensable. It is further 

 desirable that it should have a sufficiently high specific 

 efficiency to allow of its being used in a readily diffusible dilu- 

 tion ; that it should yield a cheap solution or emulsion, not 

 act on metals, and be neither caustic nor toxic. Some dis- 

 fectant substances may now be considered more in detail. 



Acids. All acids have disinfectant action, and their 

 relative values are interesting in the respect that for them 

 a general law has been fairly well established by Von 

 Lingelsheim, and confirmed by Boer namely, that the effi- 

 ciency varies with the degree of acidity. Solutions of acids 

 not of equal percentage concentration, but of equal acidity, 

 have approximately the same disinfectant efficiency what- 

 ever may be the acid, and whether it be inorganic or organic. 



The acids have no great practical application in dis- 



