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1. The virtual sterilization time (v.s.t.) has been used as a measure of the rate of disinfection of Bact. coli cultures by phenol under carefully standardized conditions. The relationship of this time to temperature at each of five phenol concentrations has been examined from a fresh point of view, since it has previously been shown (Jordan & Jacobs, 1946) that none of the commonly accepted temperature coefficients was satisfactorily constant.
2. The relationship is such that a minimum or threshold temperature exists for each concentration. A corresponding ‘maximum’ temperature has been fixed, defined as the temperature at which the v.s.t. is 10 min. The value of (v.s.t. – 10) thus varies from infinity to zero between these temperature limits.
3. Sigmoid curves are obtained when log (v.s.t. –10) is plotted against temperature for given phenol concentrations. These may be regarded as asymptotic to ordinates at the minimum and maximum temperatures.
4. The Pearl-Verhulst logistic equation gives a curve of the required sigmoid type, and this formula has been shown to fit the curves of log (v.s.t. – 10) against temperature very satisfactorily over the range of concentrations studied.
5. One of the constants of this formula partakes of the nature of a temperature coefficient, and it has, therefore, been possible to derive a truly constant temperature coefficient for each phenol concentration.
6. The values of this new temperature coefficient do not vary greatly with phenol concentration within the range studied, but it is not yet possible to establish whether it is essentially constant for all phenol concentrations with which it is possible to work.
