TESTING OF ANTISEPTICS. 827 



Action of formaldehyde.— The commercial formaldehyde contains 

 approximately 40 per cent, of the active constituent. The 10 per 

 cent, solution is prepared by adding 10 c.c. of formalin to 30 c.c. of 

 sterile water. 5 c.c. of this solution are placed in a sterile Esmarch 

 dish or test-tube and an equal volume of the suspension is then added. 

 At intervals of 1, 3, 5, 10, 15, 30 and 60 minutes a loopful of the mixture 

 is transplanted to bouillon. 



In the above method of testing no account is taken of the numer- 

 ical decrease of the organisms present. This, however, can readily be 

 done by making agar Petri dishes at the intervals given above. The 

 plates are placed in the incubator and the colonies that develop can 

 then be counted. 



Testing of Antiseptics. 



The first action of a chemical substance, when added to 

 a recently inoculated culture medium, is to inhibit the 

 growth of the organism. If the chemical substance is highly 

 poisonous and is present in su^cient amount it will event- 

 ually kill the bacteria present. On the other hand, many 

 weak substances, commonly designated as preservatives, 

 will prevent the development of bacteria, but are not able 

 to destroy them. It is necessary, therefore, to clearly un- 

 derstand the distinction between an antiseptic and a germicide. 

 The latter kills a growth, whereas the former prevents its 

 further development. A germicide, when sufficiently dilu- 

 ted, may act as an antiseptic. When an agar tube, inocu- 

 lated with the spores of the hay bacillus, is exposed to an 

 atmosphere of sulphur dioxide it becomes cloudy, and if it 

 is then placed in the incubator no growth will result. Ap- 

 parently the organism has been killed by the sulphur diox- 

 ide. If, however, a platinum wire is passed over the sur- 

 face of the cloudy agar and is then rubbed over a fresh 

 agar tube, the latter will show in a few hours an abundant 

 growth. Enough sulphur dioxide was dissolved in the first 

 agar to prevent the growth of the organisms on its surface. , 

 It was not strong enough, however, to destroy them. 



