26O 



AIDS TO BACTERIOLOGY 



in the first column and a negative result in the last. The 

 strength or efficiency of the disinfectant is expressed in 

 multiples of carbolic acid performing the same work 

 i.e., when a dilution of the disinfectant has been obtained 

 which does the same work as the standard carbolic acid 

 dilution, the former is divided by the latter, and so a ratio 

 is obtained which the authors call the ' carbolic acid 

 coefficient.' 



The use of the term ' carbolic acid coefficient ' in con- 

 nection with later tests, giving different results, has led to 

 much confusion. To guard against this, it is advisable 

 to express the results as ' Rideal- Walker ' coefficients 

 when employing the original test. 



The following table shows the degree of refinement to 

 which this test may be carried with a little care: 



B. TYPHOSUS (KRAL), TWENTY-FOUR HOURS' BROTH 

 CULTURE AT 37 C. 



Room-Temperature, 15 to 18 C. 



From this it is seen that a 1 in 70 solution of disinfectant 

 W has the same action as a 1 in 80 solution of carbolic 

 acid. Disinfectant W is therefore not quite so active 

 as carbolic acid, and this is represented by the carbolic 

 acid coefficient viz., g- = 0'87. 



Notes on the Test. For rough work it may be taken 

 that 110 parts by weight of acidum carbolicum lique- 

 factum (B.P.) contains 100 parts by weight of absolute 

 phenol. Carbolic acid sometimes contains alcohol, which 



