758 A MANUAL OF BACTERIOLOGY 



chloride. Mercuric chloride dissolved in absolute alcohol 

 has little or no efficiency, and the addition of sodium 

 chloride reduces its activity. Organisms in masses are 

 less readily acted upon by antiseptics than when they 

 are isolated. 



The efficiency of a germicidal salt in solution seems to 

 vary with its ionisation (p. 66), and the greater the disso- 

 ciation the more active will the substance be as a germi- 

 cide. Taking mercuric chloride, bromide and cyanide, 

 the ionisation of the chloride is greatest, and that of 

 the cyanide is least, and the following results show that 

 the germicidal power of the three is in this order : l 



Number of 

 colonies which developed. 



After After 



Solution. 20 minutes' 85 minutes' 



treatment, treatment. 



1 rnole HgCl 2 in 64 litres . . 7 



1 HgBr 2 . . 34 



1 ,, Hg(CN) a in 16 litres . 8 33 



Since the amount of this dissociation may be greatly 

 influenced by the presence of other substances, much 

 caution should be exercised in adding salts, etc., to 

 increase solubility or prevent precipitation, as the addition 

 may seriously impair germicidal or antiseptic power (see 

 pp/759, 765). 



The disinfection process is a gradual one. In the early 

 stages of disinfection large numbers of organisms are 

 killed, but the rate of killing becomes slower and slower 

 as time elapses. Madsen and Nyman and Miss Chick 2 

 have found that if the results be plotted, ordinates repre- 

 senting the numbers of surviving bacteria, and abscissae 



1 Findlay, Physical Chemistry, 1905. 



2 Journ. of Hygiene, vol. viii, 1908, p. 92 (Summary and Bibliog.). 



