THE EFFICIENCY OF DISINFECTANTS 103 
EXERCISE XLII 
DETERMINING THE EFFICIENCY OF DISINFECTANTS 
157. The efficiency of the more commonly used disinfect- 
ants has been determined for most of the pathogenic bacteria, 
but new disinfectants are constantly being put upon the mar- 
ket, and before it is safe to use or recommend them, their effi- 
ciency should be determined. With many of the disinfectants, 
such as carbolic acid, corrosive sublimate, lime, and the min- 
eral acids, much stronger solutions are commonly used than 
are actually necessary to kill the bacteria, owing to the fact 
that frequently it is necessary to allow for an indefinite waste 
due to the union of the disinfectant with other substances, 
usually organic, with which the bacteria are mixed. For the 
different methods of testing the efficiency of disinfectants, see 
text-books. A very simple process is given here." 
It may be desirable for students to work in groups of two or 
more in order to economize in the number of tubes required. 
If possible, however, each student should make all of the tests. 
158. Work for this exercise. Put 10 cc. of a 2% solution 
of carbolic acid, prepared from sterile distilled water, into each 
of 2 sterile test tubes. Add to one of these tubes, by means of 
a sterilized pipette, .25 cc. of a bouillon culture of B. cof or 
B. typhosus. To the other tube add a like quantity of a 
suspension in bouillon or sterile water of an agar culture of 
B. subtilis (furnished). 
Inoculate a tube of bouillon containing fully 7 cc. with 6 
loopfuls from each of these tubes after the expiration of the 
following periods of time: 1 minute, 5 minutes, 10 minutes, 
and 30 minutes. In making these inoculations allow the loop 
1 The student is referred to Bulletin No. 82. The Standardization 
of Disinfectants, Hygienic Laboratory, Public Health and Marine- 
Hospital Service. Washington, D.C. 1912. 
