318 REAGENTS AND PROCESSES 



tion should again be neutralized, if necessary, with carbonate 

 of soda. When the solution stands at a temperature of about 

 50 C., an egg stirred up in 100 gm. of water is added while 

 the mixture is stirred with a glass rod. The mixture is then 

 kept at the boiling-point for about ten minutes. This coagulates 

 the egg-albumen and clarifies the liquid. The clarified liquid 

 is now filtered by means of a hot-water funnel or while kept 

 warm in an incubator, the high temperature being necessary 

 for the reason that the mixture would become stiff at a low 

 temperature, and so incapable of being filtered. The medium 

 should be distributed while warm in sterilized test-tubes or flasks, 

 which are then stoppered with baked cotton plugs. It should 

 then be subjected to a temperature of 100 in the steam sterilizer 

 for ten minutes at four successive intervals of twenty-four hours. 

 For the reason that gelatine loses its power of solidifying at 

 ordinary temperatures after being subjected to the temperature 

 of boiling water for a long period, the time of each sterilization 

 is necessarily reduced to about ten minutes and the number of 

 sterilizations is increased to four; whereas, with other solidifying 

 substances, such as agar-agar, the length of each sterilization 

 may extend to one hour, and the number of sterilizations need 

 be only two or three. 



In pouring the filtered medium into the test-tubes care should 

 be taken not to get any of the medium on the upper portion 

 of the tube where the cotton plug would be likely to come in 

 contact with it, else the plug would later be difficult of removal. 



A solid nutrient medium which will remain solid at a higher 

 temperature than the gelatine medium may be prepared from 

 agar-agar, a substance obtained from certain gelatinous algae, 

 as follows: Two gm. of the agar are broken into small pieces 

 and soaked in cold water for twenty-four hours. Then the 

 water is poured off and the swollen agar is added to i liter 

 of the peptonized meat infusion. The mixture is boiled for 

 several hours until the agar is completely dissolved. The solu- 

 tion is then neutralized with a solution of carbonate of soda, 

 filtered, distributed in flasks or test-tubes, and sterilized by 



