Testing Germicidal Value of Liquids 269 



1. The test dilutions are made 20 per cent, stronger to allow for 

 the dilution made by the addition of the solution of organic matter. 



2. An organic matter solution is to be prepared. It consists of 

 water containing 10 per cent, of peptone and 5 per cent, of gelatin. 

 The solids are dissolved and the solution sterilized. Titration is 

 not essential. 



The variations in technic are simple. Of the dilutions made 

 20 per cent, stronger than for the other experiment, 4 cc. (not 5 cc.) 

 are measured into each seeding tube. The culture after being fil- 

 tered is added to the organic matter in the proportion of o.i cc. to 

 each I cc. to be employed in seeding. The addition of i.i cc. of the 

 organic solution culture mixture to each seeding tube, gives a total 

 of 5 cc. of diluted disinfectant containing o.i co. of culture and a 

 total of 2 per cent, of peptone and i per cent, of gelatin. Except 

 for the sUght difference in the dilutions and the seeding with mixed 

 culture and organic fluid the method is the same, and the method 

 of calculating the results is the same. 



Anderson and McClintic point out that it is manifestly cheaper 

 to purchase a disinfectant for 60 cents a gallon than to purchase one 

 for 30 cents a gallon, providing the; former has four times the effi- 

 ciency of the latter. The true cost of a disinfectant can only be 

 determined by taking into consideration the phenol coefficient and the 

 cost of the disinfectant per gallon. The cost of a disinfectant 

 per 100 units of efficiency as compared with pure phenol is obtained 

 by first dividing the cost per gallon of the disinfectant by the cost 

 per gallon of pure phenol; the efficiency ratio is of course obtained by 

 dividing the coefficient of the disinfectant by the coefficient of phe- 

 nol, but as the coefficient is always i, the efficiency ratio is repre- 

 sented by the phenol coefficient of the disinfectant. 



The cost ratio divided by the efficiency ratio (the coefficient of the 

 disinfectant) gives the cost of the disinf ectan.t per unit of efficiency as 

 compared with the cost per unit of efficiency of pure phenol = 1. 

 By multiplying by 100 the relative cost of 100 units is obtained thus: 



Cost of disinfectant Coefficient of disinfec- 



_per gallon^ ^^^^ ^ ^ tant. ( = Efficiency 



Cost of phenol per ' Coefficient of phenol. ratio.) 



gallon. ( = 1.) 



= cost of the disinfectant per unit of efficiency as compared with 

 phenol = 1, and by multiplying by 100 the cost of 100 units is 

 obtained. For instance, the cost of disinfectant "Can" is $0.30 

 per gallon and it has a coefficient of 2.12; the cost of phenol is $2.67 

 and it has a coefficient of i. Then, 

 0.30 2.12 



; i = 0.052 



2.67 I ^ 



Therefore, the comparative cost per unit of efficiency of " Can" and 

 phenol respectively, is as 0.052:1; or, by multiplying by 100, 

 the relative cost per 100 units — 5.2 : 100 is obtained. 



