This constant having been fixed, prepare various solutions of germicide-x with sterilised distilled water by accurate volumetric methods, commencing with a solution somewhat stronger than that representing the inhibition coefficient. The solutions must be prepared in fairly large bulk, not less than 5 c.c. of the disinfectant being utilised for the preparation of any given percentage solution. _b. Standard Control._--~Phenol.~ The standard germicide used for comparison should be one which is not subject to variation in its chemical composition, and the one which has obtained almost universal use is Phenol. The following table shows the effect of different percentages of carbolic acid upon B. coli for varying contact times, compiled from an experiment conducted under the standard conditions referred to under Environment. The results closely correspond to those recorded by the Lancet Commission on Disinfectants, 1909. ---------------------+----------------------------------- | Contact time in minutes. Percentage of phenol +------+---+---+---+---+---+---+---- | 2-1/2| 5 |10 | 15| 20| 25| 30| 35 ---------------------+------+---+---+---+---+---+---+---- 1.20 | - | - | - | - | - | - | - | - 1.10 | - | - | - | - | - | - | - | - 1.0 | + | - | - | - | - | - | - | - 0.9 | + | - | - | - | - | - | - | - 0.85 | + | + | - | - | - | - | - | - 0.80 | + | + | + | - | - | - | - | - 0.75 | + | + | + | + | + | - | - | - 0.7 | + | + | + | + | + | + | - | - 0.65 | + | + | + | + | + | + | + | - ---------------------+------+---+---+---+---+---+---+---- - = No growth, i. e., bacteria killed. + = Growth, i. e., bacteria still living. From this it will be seen that the following percentage solutions will need to be prepared, namely: 1.1 per cent., 1.0 per cent., 0.9 per cent., 0.75 per cent., 0.7 per cent., as controls for each experiment. Prepare solutions of varying percentages by weighing out the quantity of carbolic acid required for each and dissolving in 100 c.c. of pure distilled water in an accurately standardised measuring flask. The solutions must be prepared freshly as required each day. ~Environment.~-- _a. General._-- Close the windows and doors of the laboratory in which the investigation is carried out, to avoid draughts. Flush over the work bench and adjacent floor with 1:1000 solution of corrosive sublimate. Caution the assistant, if one is employed, to avoid unnecessary movement or speech. _b. Contact Temperature_, ~15-18° C.~--