BY R. GREIG-SMITH. 321 



Tt is customary, when comparing the germicidal properties of 

 some disinfectants, to refer to them in terms of phenol, which 

 for many reasons is taken as a standard. The phenol-coefficient 

 is obtained by dividing the weakest dilution of a disinfectant 

 which destroys a bacterium in a certain time and at a certain 

 temperature by the weakest dilution of phenol lethal in the same 

 time and at the same temperature. In Table ii. the coefficients 

 have been obtained by dividing the probable effective dilutions 

 of cineol by those of phenol. 



The coefficiency curve is comparatively high and reaches a 

 maximum of 3 - 4 in thirty minutes, and slowly falls to 2*8 in four 

 hours. As Delepine* reminded us, the ratio between the lethal 

 doses of two disinfectants is not the same for exposures of dif- 

 ferent durations. 



There is no definite rule as to the exposure which should be 

 taken in calculating the phenol coefficient of any disinfectant. 

 The method suggested by Rideal and Walker is to withdraw 

 portions of the tests every two and a half minutes up to fifteen, 

 and to take the dilutions of the disinfectants which are lethal at 

 any one exposure. Thus the coefficient might be calculated from 

 any 2| minute period up to 15 minutes. Sims Woodheadf deter- 

 mined the coefficients at 1\ and at 30 minutes and took the 

 average. BlythJ used the 12^ minute interval. Delepine§ pre- 

 ferred exposures of not less than 20 minutes for bacteria such as 

 B. coli communis, and took 20 or 30 minutes as the proper 

 exposure, but he recognised that half-hourly and hourly exposures 

 had the advantage of giving more steady results. Chick and 

 Martinj[ considered that 30 minutes was best. A study of the 

 coefficients of cineol leads one to the same conclusion. It would 

 clearly be unwise to take a time during the rapid rise in the 

 cineol curve. It should rather be taken when the curve has 

 begun to assume its nearly horizontal position. The curves for 



* Journ. Soc. Chem. Ind., 1910, 1344. 



t Lancet, 1909, 1454. 



£ Journ. Soc. Chem. Ind., I'll Ml, 1183. 



§ Idtm, 1911, 334. 



Journ. Hyg., 1908, 654. 



25 



