Quantitative observations were made on the rate of disinfection of Bacterium coli and of the spores of Bacillus subtilis by caustic soda at different temperatures and different concentrations. The tests were carried out in the presence of 1/1000 milk. It was found that:

(1) With Bact. coli the concentration coefficient n was about 2·7, and the temperature coefficient θ10°C. about 2.

(2) With B. subtilis spores the concentration coefficient n was about 1·75 and the temperature coefficient θ10°C. about 1·5.

(3) With Bact. coli the values for the reaction velocity constant k tended to be irregular at 40° C. and with B. subtilis spores at 70° C, suggesting that at these temperatures some additional factor, presumably heat coagulation of the protein, was beginning to affect the results.

(4) With both organisms the value of the reaction velocity constant k was relatively slow at the start and tended to increase progressively during the course of disinfection. Whether it diminished again as sterility was approached could not be ascertained for technical reasons.

(5) With both organisms the presence of 1/1000 milk did not seem to affect the rate of disinfection as compared with distilled water.

(6) One experiment with Bact. coli suggested that the rate of disinfection was affected appreciably by the number of organisms present. Increasing the number of organisms in the suspension 100 times diminished the value of k by about one-third.

(7) The value of k was about 3,000,000 times greater with Bact. coli than with B. subtilis spores.

(8) If the mean value of n is taken as 2·2, it follows that doubling the concentration of caustic soda increases the reaction velocity by 4·6 times. If, for example, with a given concentration of caustic soda sterility was reached in 9 min., then doubling the concentration would reduce this time to 2 min., while halving the concentration would increase it to about 40 min.

(9) If the mean value of θ10°C. is taken as 1·75, it follows that a rise of 10° C. in the temperature increases the reaction velocity 1·75 times. If, for example, with a given temperature sterility was reached in 9 min., then raising the temperature 10° C. (18° F.) would reduce this time to about 5 min., while lowering the temperature 10° C. would increase it to about 15 min. With temperatures, however, of over 40° C. (104° F.) the rate of destruction of yegetative organisms, and with temperatures of over 70° C. (158° F.) the rate of destruction of sporing organisms in the presence of caustic soda is probably increased by the effect of the heat itself.

(10) A comparison of our figures for caustic soda with those of Chick for phenol shows that a 0°05% solution of caustic soda at 20° C. destroyed Bact. coli about five times as fast as a 0°5% solution of phenol at 20° C. destroyed Bact. paratyphosum B, and that a 5% solution of caustic soda at 30° C. destroyed B. subtilis spores nearly three times as fast as a 5 % solution of phenol at 33·3° C. destroyed anthrax spores. The superiority of caustic soda over phenol, particularly at concentrations likely to be used in practice, is manifest.

(11) Discussing the standardization of disinfectants, we conclude that the value of k taken in the middle stage of the reaction, or from the beginning of disinfection to about the end of the middle stage, affords the most suitable measure of comparison. This is essentially the same conclusion as that reached by Withell (1942), who uses as his index the time necessary to destroy 50% of the organisms.