TO DETERMINE THERMAL DEATH-POINT. 243 



Sternberg suggested a method in which the time should 

 remain constant (two hours' exposure), and the object be 

 the determination of the exact dilution of the reagent 

 required to destroy the bacteria. " Instead of subjecting 

 a few of the test-organisms attached to a silk thread to 

 the action of the disinfecting agent, a certain quantity of 

 the recent culture — usually 5 c.cm. — has been mixed 

 with an equal quantity of a standard solution of the 

 germicidal agent, . . . and after two hours' contact one 

 or two ose-fuls would be introduced into a suitable nutri- 

 ent medium to test the question of disinfection." 



A very simple and popular method of determining the 

 germicidal value is to make a series of dilutions of the 

 reagent to be tested ; add to each a couple of loopfuls of 

 a fresh liquid culture, and at varying intervals of time 

 transfer a loopful to fresh culture-media. By a little 

 ingenuity this method may be made to yield information 

 as to both time and strength. 



When it is desired to secure information concerning 

 the progress of the germicidal action of reagents, body- 

 fluids, etc., especially in the unusual and interesting 

 cases in which the material subjected to the test may 

 exert a restraining action for a time only, or bring about 

 destruction of some or many, but not all of the germs, 

 the use of the Petri dish can be introduced. 



For example, it is desired to determine whether a 

 blood-serum is germicidal or not. Into about 5 c.cm. of 

 the serum contained in a test-tube, two or three loopfuls 

 of any desired bacterium, in liquid culture, are added. 

 The tube is well agitated and immediately one loopful is 

 transferred to a tube of melted gelatin, distributed 

 through it, and poured into a Petri dish. After one 

 minute the operation is repeated, in five minutes again, 

 and so on as often as is desired. 



The dishes are stood away until the bacteria develop 

 into colonies, which are then counted with a Wolfhiigel 

 apparatus. On the first dish there may be 100 colonies; 

 on the second, 80; on the third, 50; on the fourth, 20; on 



