48 LABORATORY EXERCISES IN BACTERIOLOGY. 



same medium, diffusing it by gentle agitation, and then planting therein from a virulent 

 culture of the germ employed in the test, so as to note whether such proportion exercises 

 any destructive or restraining influence. 



Exercise 16. Prepare four tubes of one per cent, peptone solution, 

 each containing four cubic centimeters; inoculate each with the colon 

 bacillus and incubate for twenty-four hours, protecting the contents of 

 the tubes against evaporation by a rubber cap or rubber stopper over the 

 usual cotton plug. Having the following day made certain of the success 

 of the infection of each tube, add to the first one cubic centimeter of a 

 five per cent, solution of carbolic acid (thus making a one per cent, dilu- 

 tion of the acid in this tube) ; to the second, one cubic centimeter of a ten 

 per cent, solution (two per cent, solution in the tube) ; to the third, one 

 cubic centimeter of a fifteen per cent, solution (a three per cent, solution 

 in tube); and to the fourth, one cubic centimeter of a twenty per cent, 

 solution (four per cent, in the tube). Diffuse the disinfectant immediately 

 in each by gentle agitation. After appropriate intervals (five, ten, twenty, 

 thirty, forty, fifty, and sixty minutes) transfer with the platinum loop 

 one loopful from each tube to a fresh tube of the peptone solution. Each 

 tube, properly marked, is incubated, and, at the usual intervals of one, 

 two, and three days, observations are made and recorded. What tubes 

 permit growth? What tubes show no development? That tube planted 

 from the greatest dilution of the carbolic acid, having had the shortest 

 action period, which shows no growth, may be taken as representing the 

 disinfectant value of this substance for the colon bacillus. To verify 

 the result, make a preparation of the same degree of dilution of the acid 

 in a sterile tube of the peptone solution, and inoculate it with a loopful 

 of a virulent culture of the organism. If growth readily follows, it may 

 be inferred that no material influence was exercised by the small amount 

 of carbolic acid transferred in the test experiment; if growth occurs but 

 slowly, retardation only may have been accomplished, and in such event 

 the contents of the tube accepted as indicating the disinfecting power 

 of the acid in the test experiment should be diluted freely with sterile 

 peptone solution, further incubated, and observed. If no growth should 

 follow, however, it may be accepted for practical purposes that actual 

 disinfection, and not mere retardation (antisepsis), was accomplished. 



Determination of Antiseptic Values. It should be recalled here, as in the case 

 of disinfection, that the antiseptic value must vary for every substance according to the 

 microorganism subjected to its influence, and probably according to the character of the 

 surrounding medium as well. The method for determination is a simple one, consisting of 

 the addition of known amounts of the material under investigation to known amounts 

 of sterile nutrient medium (one per cent, peptone solution), each tube thus prepared 



