244 



BACTERIOLOGICAL METHODS 



disinfectant that sells for 60 cents a gallon than one that sells for 

 30 cents a gallon, if the former has four times the efficiency of the 

 latter. 



The true cost of a disinfectant can be determined only by 

 taking into consideration the phenol coefficient and the cost per 

 gallon of the disinfectant. 



The following table (IV) is a good illustration of the value 

 of a determination of the comparative cost per 100 units of 

 disinfectant in terms of 100 units of pure phenol: 



Table IV 



It will be seen that the substance Chi has a higher coefficient 

 than any of the others in the table, but its high cost per gallon 

 results in its being placed second in cost per loo units. 



The cost per loo 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; this gives 

 the price ratio between the disinfectant and pure phenol; the 

 cost ratio is then divided by the phenol coefficient, which gives us 

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

 The cost per unit is then multiplied by loo to give the cost per 

 100 units. 



The Ohno-Hamilton Phenol Coefficient 



Tatsuzo Ohno and H. C. Hamilton of the Parke, Davis Re- 

 search Laboratory have proposed a method for the bacteriological 



