118 ATLAS OF BACTERIOLOGY. 



off on a millimetre scale, at the temperature of the 

 room, the rate at which the liquefaction of the gelatin 

 proceeds for days and weeks. Qualitative tests may 

 be simply made by using 1 c.c. of a liquefied gelatin 

 culture which has been sterilized with carbolic acid. * 

 This material also suffices in testing the influence of 

 the nutrient medium upon the formation of the fer- 

 ment. By this method we may also compare the 

 action of different degrees of concentration of differ- 

 ent bacterio-trypsins. The less the percentage of 

 gelatin and the nearer the temperature to incubating 

 temperature, the more certainly do we obtain the 

 action of even traces of ferment. In such critical 

 cases the experiment is continued for two weeks and 

 we then note whether the test tubes in the refrigera- 

 tor, provided with the ferment, remain fluid, while 

 the control tubes remain rigid. 



In order to demonstrate the production of a true 

 peptone, we proceed in the following way : 



The variety of bacteria in question is cultivated 

 upon a fluid albuminous nutrient medium free from 

 peptone (blood serum, milk serum, milk). If the 

 culture grows well, all the albuminoids, with the 

 exception of the peptone, are precipitated by the ad- 

 dition of solid ammonium sulphate (about 30 gm. to 

 20 c.c.). Milk and milk serum may be warmed to 

 60-80, blood serum to about 40. The precipitate 

 is then filtered, the filtrate cooled; a part is made 

 strongly alkaline by the addition of potash, and one- 

 per-cent solution of copper sulphate is then added 



* As a matter of course we must never fail to make a control 

 test with two-per-cent solution of carbolic acid in water (free 

 from germs) . 



