332 Mr. C. H. Browning and others. Relationships between 



clear to the naked eye. Accordingly, in all cases the presence or absence 

 of living organisms was decided by subculturing each mixture on nutrient 

 agar, which was then incubated for 48 hours at 37° C. The results are 

 recorded numerically, the highest concentration which permitted vigorous 

 growth, and the lowest producing sterility, as tested by subculture of a loop- 

 ful of the mixture on agar, being given. In certain cases there is a wide 

 zone separating these two concentrations, which indicates that the particular 

 compounds are specially bacteriostatic in action, and that concentrations 

 considerably less than that required to kill the bacteria still have the effect 

 of restraining growth. It is to be noted that more useful information is 

 obtained by making subcultures from the mixtures of antiseptic and 

 organisms on a solid medium than in a fluid one, as in the latter case it is 

 not possible to determine any degree of action of the antiseptic short of 

 complete sterilisation. The investigation has been complicated by such 

 questions as effects due to differences in solubility and variations in dis- 

 sociation, leading possibly to differences in hydrogen-ion concentration of 

 the solutions. Thus it has been found, when examining particular com- 

 pounds, that comparatively small variations in hydrogen-ion concentration 

 may exercise a great influence on the antiseptic potency (Browning, 

 Gulbransen and Kennaway, 6). With diaminoacridine methochloride in 

 peptone water, the concentration required to sterilise B. coli when the Pn 

 value of the solution lay between 4 and 5 was 1 : 2000 ; within a range from 

 6 to 7, 1 : 10000 of the dye sufficed ; at 8 to 9, 1 : 40000 sterilised, while 

 at 11 a concentration of 1 : 200000 was sufficient. In each case the 

 medium with similar reaction, but without the antiseptic, permitted vigorous 

 growth of the organisms. In addition, there is the further factor of 

 variability in the behaviour of the bacterial culture. With regard to the 

 latter, it appears to be highly probable, if not definitely established, that the 

 individuals in a given culture are not all equally susceptible to harmful 

 influences ; thus, irregularities are observed when a particular concentration 

 of antiseptic is caused to act on duplicate samples of the same infected 

 material. This has been drawn attention to by Eichet and Cardot (9), and 

 has been observed also in our own work. Further, the occurrence of 

 variations in the culture from time to time can scarcely be excluded 

 although no evidence has been obtained pointing to permanent or to 

 regularly cyclic changes. Eepeated series of tests carried out with a view 

 to examining the action of diaminoacridine methochloride on a single strain 

 of B. coli in ox serum have shown the following variable results. A con- 

 centration of 1 : 1000000 and upwards, sterilised in 8 series, 1 : 400000 

 and upwards, sterilised in 17 series, 1:200000 and upwards in 16 series, 



