HYGIENIC LABORATORY METHOD 157 
matter, and therefore serves as a fairly accurate means of estimate 
ing, in the presence of organic matter, the germicidal values of dis- 
infectants in general. 
The method of determining the coefficient of disinfectants with 
the addition of organic matter is identical in many respects .with the 
method in which no organic matter is used, the former differing from the 
latter principally in the strength of the disinfectant dilutions that 
have to be prepared and in the prepa- 
ration and addition of the organic 
matter to the disinfectant dilutions 
when performing the test. As the 
method in which no organic matter 
is added has already been given in 
detail, further description here will 
consist only of the differences between 
the two methods. 
Dilutions. In making the dilutions 
of a disinfectant for determining its 
coefficient in the presence of organic 
matter, allowance must be made for the 
further dilution of the disinfectant when 
the volume of organic matter is added 
thereto. For instance, if 1 c.c. of 
organic matter is added to 5 c.c. of a 
1 per cent dilution of a disinfectant the 
percentage strength of the disin- 
fectant is proportionately reduced, it 
then being about 0.83 per cent. Ni ee Ns 
The volume of organic matter that p,, 4g _water Bath Showing Po- 
can be added to the disinfectant sition of Holes for Seeding Tubes 
dilution is, of course, variable, as and Thermometer in Place. 
is also the volume of disinfectant 
dilution to which it can be added. Consequently, we have decided 
rather arbitrarily to use the organic matter by adding 1 c.c. of it to 
4 cc. of the disinfectant dilution contained in a seeding tube. It 
will be seen that the strength of the disinfectant is reduced 20 per 
cent by the addition of the organic matter and that the dilutions of 
the disinfectant must be made accordingly. For example, if it is 
desired to test a 1 per cent strength of a disinfectant it is necessary 
to prepare a strength of 1.25 per cent, 4 ¢.c. of which becomes.a 1 per 
cent strength when 1 c.c. of the organic matter is added to it. 
old 
