THE DESTRUCTION OF BACTERIA 83 



without labelling. The subculture tubes are incubated for 48 hours at 

 37° C. and those in which growth is observed are recorded positive. 



To obtain the coefficient the weakest dilution of the unknown 

 antiseptic which kills in 2)4, minutes is divided by the weakest dilution 

 of phenol which kills in the same time. The same is done for the weak- 

 est strength that kills in 15 minutes and an average is taken. The 

 results of such a test are shown in the table on page 82. 



As only the 2J^-minute and 15-minute intervals are used in deter- 

 mining this result it seems unnecessary to make plants at the intervening 

 periods except in special cases where more detailed information is desired. 



The procedure may be modified by adding some organic substance 

 such as killed bacteria to the diluted antiseptic. For many substances, 

 e.g., bichloride of mercury, the antispetic value in presence of organic 

 matter is much lower than in watery solution. Anderson and McChntic 

 insist that great care in making the dilutions and rigid adherence to a 

 uniform technique are necessary to obtain consistent results in 'such tests. 



Determination of Antiseptic Values. — The antiseptic or in- 

 hibitive strength of a chemical substance, sometimes spoken of 

 as the "coefficient of inhibition," is determined by adding to 

 definite quantities of a given culture medium, graded percent- 

 ages of the chemical substance which is being investigated and plant- 

 ing in these mixtures equal quantities of the bacteria in question. 

 The medium used for the tests may be nutrient broth or melted gelatin 

 or agar. If broth is used, growth is estimated by turbidity of the 

 medium and by morphological examination; if the agar or gelatin is 

 employed, plates may be poured and actual growth observed. 



Thus, in the case of carbolic acid, a 5 or 10 per cent solution is 

 prepared and added to tubes of the medium, as follows: 



Tube 1 contains 5% carbolic 2 c.c. + broth 8 c.c. = 1: 1,000 carbolic acid. 



1 c.c. + broth 9 c.c. = 1:200 " " 



.5 c.c. + broth 9.5 c.c. = 1:400 " " 



" 7 c.c. + broth 9.8 c.c. = 1:1,000 " " 



.1 c.c. + broth 9.9 c.c. = 1:5,000 " " 



To each of these tubes a definite quantity of the bacteria is added 

 either by means of a standard loopful of a fresh agar culture, or better by 

 a measured volume of an even emulsion in sterile salt solution. The 

 inoculated tubes are then incubated at a temperature corresponding to 

 the optimum growth temperature for the microorganism in question. 

 The tubes are examined for growth from day to day. From tubes 

 containing higher dilutions, in which no growth is visible, transplants 



