GEORGE F. REDDISH 309 



the antiseptic salve or ointment which is in marked contrast to the turbidity of the surround- 

 ing medium caused by the heavy growth of the organism. 



In treating infected surfaces v^^ith preparations of this nature it is necessary that the 

 active ingredients leave the inert base and become free to surround the infective or- 

 ganisms. It is only in this way that the preparation will be of benefit in preventing 

 the growth of and even killing these micro-organisms. Serum agar simulates fairly 

 closely the conditions met with in skin and wounds. It is permeable, semi-solid, iso- 

 tonic, and constitutes a valuable laboratory means of approximating the conditions, 

 found in human and animal tissues, at least so far as the preparations under consider- 

 ation are concerned. 



In testing bunion pads, corn plasters, surgical dressings, and powders for wound 

 dressings for which antiseptic claims are made, the procedure just given is employed. 

 These preparations are placed on top of a poured serum agar plate containing M. au- 

 reus, and incubated at 37° C. for forty-eight hours. The antiseptic in such products per- 

 meates the agar medium and prevents the growth of the test organism. The amount 

 of antiseptic contained and its solubility, together with the ease with which it leaves 

 the base, are factors in determining the width of the clear zone around these prepa- 

 rations. 



In these tests effort has been made to simulate practical conditions of disinfection 

 and antisepsis as closely as possible. In an arbitrary test it is difficult to duplicate 

 exactly the conditions found in practice, but results obtained by these methods give 

 a fair indication of the efficiency of the preparations tested. It is very important that 

 the method of testing should suit the preparation being examined. 



