GERMICIDAL POWER OF CHEMICAL DISINFECTANTS 197 



organic matter is used, the former differing from the latter principally in the 

 strengths of the disinfectant dilutions that have to be prepared and in the 

 preparation 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 only 

 consist 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 i cc. of organic matter is added to 5 cc. of a i per cent, dilution of a 

 disinfectant the percentage strength of the disinfectant is proportionately 

 reduced, it then being about 0.83 per cent. 



The volume of organic matter that can be added to the disinfectant dilu- 

 tion is, of course, variable, as 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 i cc. of it to 4 cc. of the disinfectant dilution con- 

 tained 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 i per cent, strength of a disinfectant it is necessary to prepare a strength 

 of 1.25 per cent., 4 cc. of which becomes a i per cent, strength when i cc. of the 

 organic matter is added to it. 



We also tried adding 2.5 cc. of the organic matter to 2.5 cc. of the disinfect- 

 ant dilution, but found it rather difficult and cumbersome to do, particularly 

 when the experiment has to be performed with a number of different strengths of 

 the disinfectant. When using the proportions as just stated, the dilutions of the 

 disinfectant are made double the strengths it is desired to test, thus allowing for 

 the further dilution when an equal volume of organic matter is added thereto. 



In using the proportions of i cc. of organic matter to 4 cc. of the disinfectant 

 dilution we have found the following tables (18 and 19) of service in preparing 

 the dilutions of the disinfectant. 



TABLE 18. STOCK 5 PER CENT. SOLUTION 

 [5 cc. of disinfectant + 95 cc. distilled water = solution A] 



Strength Strength Cc. Cc. of Cc. Cc. of 



to be tested to be made of A dist. water of A dist. water 



1:30 1:24 = 20 + 



1:35 i:28 = 20 + 



i : 40 1:32 = 20 + 



1:50 1:40 = 20 + 



1:60 1:48 = 20 + 



1:65 1:52 = 20 + 



i: 70 1:56 = -20 + 



1:70 1:56 = 10 + 



