776 



A MANUAL OF BACTERIOLOGY 



the second set for five minutes, and so on. The results (taken 

 from an actual test) may be charted as follows : 



B. typhosus, 24-hour broth culture at 37 C. 

 Room-temperature 60 F. 



-j- = growth in the subcultures. * = no growth in the subcultures. 



From this it will be seen that the disinfectant X in a solution 

 of 1 in 1,600 kills in the same time (ten minutes) as carbolic 

 acid 1 in 100. This result is expressed as a coefficient obtained 

 by dividing the strength of disinfectant by the strength of carbolic 

 which kills each in the same time ; in the present instance the 

 coefficient is V 6 A = 1 6 '0, and this figure is known as the " car- 

 bolic acid coefficient." 



If nothing is known concerning the germicidal strength of the 

 disinfectant, some preliminary experiments should be performed 

 with dilutions at wide intervals as regards strength (e.g. 1-100, 

 1-500, 1-1,000, 1-1,500, 1-2,000, etc.), and when the limit has 

 thus been approximately ascertained, the test is performed as 

 above. 



Precautions to be taken in carrying out the test. (a) The culture 

 should be a broth one about twenty-four hours old. Immediately 

 before use, it is shaken and then allowed to stand for half an hour 

 to deposit clumps. The culture is added by means of a small 

 graduated pipette or by means of a standardised dropping pipette. 

 Four drops of culture added with a pipette the point of which is 

 No. 27, Stubbs Wire gauge, is equivalent to 0-2 c.c. Eideal and 

 Walker now add 0'5 c.c. of culture to 5 c.c. of solution. 



(b) The carbolic acid (the crystals of which should have a 

 melting-point of not less than 40-5 C.) should be kept in the form 



