BACTERICIDAL PROPERTIES 21 



ciilate from the price of pure phenol which will be the 

 most economical to buy. It is apparent to any one that 

 it is better to pay sixty cents a gallon for disinfectant 

 "A" than thirty cents per gallon for disinfectant "B" 

 if "A" has four times the efficiency of "B." 



To determine the cost per 100 units of efficiency of any 

 preparation as compared to phenol, divide the cost per 

 gallon by the cost per gallon of pure phenol ; this gives 

 the cost ratio of the tw^o. The efficient ratio of the two 

 is obtained by dividing the phenol coefficient of the prep- 

 aration by the phenol coefficient of phenol, which is al- 

 ways 1, since it is the unit. The efficiency ratio is there- 

 fore always the phenol coefficient. The cost ratio divided 

 by the efficiency ratio (the phenol coefficient) gives the 

 cost of the disinfectant per unit efficiency of phenol. 

 Multiplying by 100 gives the relative cost per 100 units. 

 Thus, 



Cost of disinfectant per gallon coefficient of disinfectant 



Cost of phenol per gallon coefficient of phenol ( = 1) 



Cost of disinfectant per unit of efficiency compared to phenol 

 = 1. 

 Multiplying by 100 gives coefficient per 100 units of phenol. 



For example, the cost of carbo-campho, with which 

 most veterinarians are familiar, is $2.50 per gallon and 

 has a phenol coefficient of .57 ; the cost of phenol is $3.25 

 per gallon,^ and has a coefficient of 1. Then, 



2.50 .57 



^ =1.33. 



3.25 1 



Therefore the comparative cost of carbo-campho per 

 unit of efficiency and phenol is 1.33 :1 ; or, multiplying 

 by 100, we get 133:100, which means that $1.33 worth 



^The cost of phenol is considerably higher than this at the 

 present time, $4.95 per g-allon, due to the war conditions abroad, 

 but it was deemed best to quote the usual price rather than the 

 unnatural one due to the present war conditions. 



