CULTIVATION OF BACTERIA. 109 



Gelatin. The culture-medium known as gelatin has de- 

 cided advantages over the bouillon, not only because it is 

 an excellent food for bacteria, and, like the bouillon, trans- 

 parent, but because it is also solid. Nor is this all : it is 

 a transparent solid which can be made liquid or solid at 

 will. It is prepared as follows : To 1000 c. cm. of meat- 

 infusion or to 1000 c.cm. of water containing 2 grams of 

 beef-extract in solution, 10 grams of peptone, 5 grams of 

 salt, and 100 grams of gelatin ("Gold label " is the best 

 commercial article) are added, and boiled for about an 

 hour over a moderately hot flame. Double boilers are 

 very slow, and if proper care is exercised there is little 

 danger of the gelatin burning. It must be stirred occa- 

 sionally, and the flame should be so distributed by wire 

 gauze as not to act upon a single point of the bottom of 

 the kettle. At the end of the hour the albumins of 

 the meat-infusion will be coagulated and the gelatin 

 thoroughly dissolved. Gunther has shown that the 

 gelatin congeals better if allowed to dissolve slowly in 

 warm water before boiling. The liquid is now cooled 

 to 60 C. and neutralized i. e. alkalinized. As the gela- 

 tin is itself acid, a relatively larger amount of the sodium- 

 carbonate solution will be needed than was required for 

 the bouillon. When the proper reaction is attained, as 

 much water as has been lost by vaporization during the 

 process of boiling, intimately mixed with the white of 

 an egg, is added, well stirred in, and the whole boiled 

 for half an hour, then filtered. 



If the filter-paper be of good quality and properly 

 folded (pharmaceutical filter), and if the gelatin be prop- 

 erly dissolved, the whole quantity should pass through 

 before cooling too much. Should only half go through 

 before cooling, the remainder must be returned to the 

 pot, heated to boiling once more, and then passed through 

 a new filter-paper. As a matter of fact, gelatin generally 

 filters readily. A wise precaution is to catch the first few 

 centimeters in a test-tube and boil them, so that if a 

 cloudiness shows the presence of uncoagulated albumin, 



