]62 ANTISEPTICS AND DISINFECTANTS. 



than two weeks. Having found that 1 :400 and below prevents 

 development, and 1 . 500 does not, we may make further experiments 

 to determine the antiseptic power within narrower limits ; but this 

 is hardly necessary from a practical point of view. 



In these experiments the result will be influenced by several cir- 

 cumstances, as follows : 



(a) By the composition of the nutrient medium. This is a 

 very important factor, especially in determining the antiseptic value 

 of certain metallic salts. The presence of a considerable quantity 

 of albumin, for example, reduces greatly the antiseptic power of 

 mercuric chloride, silver nitrate, creolin, etc. The presence of a sub- 

 stance chemically incompatible, as, for example, sodium chloride in 

 testing nitrate of silver, will of course neutralize antiseptic action. 



(b) The nature of the test organism. Within certain limits an 

 antiseptic for one microorganism of this class restrains the devel- 

 opment of all, but there are wide differences in the ability of differ- 

 ent species to grow in the presence of different chemical agents. 

 Some grow readily in the presence of a considerable amount of free 

 acid, others are restrained by a slightly acid reaction of the medium 

 in which they are placed. The Bacillus acidi lactici, for example, 

 can thrive in the presence of a considerable amount of the acid 

 which is a product of its growth, but there is a limit to its power of 

 developing in the presence of this and other acids. So, too, Mi- 

 crococcus urese, which causes the alkaline fermentation of urine, 

 grows in the presence of a considerable amount of carbonate of am- 

 monia, but is finally restrained in its growth by this alkaline salt. 

 The following determinations by Boer show the difference in the 

 antiseptic power of hydrochloric acid for certain pathogenic bacte- 

 ria : Bacillus of anthrax (without spores), 1 : 3,400 ; diphtheria bacil- 

 lus, 1 : 3,400 ; glanders bacillus, 1 : 700 ; typhoid bacillus, 1 : 2,100 ; 

 cholera spirillum, 1 :5,500. It will be noted that the cholera spiril- 

 lum is restrained in its growth by about one-eighth the amount of 

 hydrochloric acid which is required to prevent the development of 

 the bacillus of glanders. The typhoid bacillus has a special tole- 

 rance for carbolic acid, etc. 



(c) The temperature at which the experiment is made. At 

 the temperature most favorable for growth a greater proportion of 

 the antiseptic agent is required than at unfavorable temperatures- 

 lower or higher. 



(d) The restraining influence for spores is much greater than 

 for the vegetative form of bacteria. 



Methods of Determining Germicide Value. The disinfecting 

 power of a chemical agent is determined by allowing it to act for a 

 given time, in a definite proportion, on a pure culture of a given 



