Testing Germicidal Value of Liquids 261 



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 beingfiltered 

 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 cc. of culture and a 

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

 for the slight 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 efSciency 

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

 mined 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 pure phenol; the efficiency 

 ratio is of course obtained by dividing the coefficient of the dis- 

 infectant by the coefficient of phenol, but as the coefficient is al- 

 ways I, the efficiency ratio is represented 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 disinfectant per unit of 

 efficiency as compared with the cost per unit of efficiency of pure 

 phenol = I. By multiplying by 100 the relative cost of 100 units 

 is obtained thus: 



Cost of disinfectant Coefficient of disinfec- 



per gallon. ( = Cost ratio) - '^^"^- ( = Efficiency 



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



gallon. ( = !•) 



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

 phenol = I, 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, 



o.so 2.12 



-^ -. = 0.052 



2.67 I -^ 



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

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

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



