METHODS OF CULTURE AND OF EXAMINATION. 81 



or in slanting layers, to be employed subsequently for stab- 

 cultures or for streak-cultures. The liquefaction-point of 

 the gelatin may be increased a degree or two if it be per- 

 mitted to stand for twenty-four hours before being used 

 (Forster). 



For special purposes (cultivation of yeast-cells), when 

 gelatin of acid reaction is required, potato- gelatin or malt- 

 gelatin may be employed. To prepare the former, 500 

 grams of cleansed, peeled, and grated potatoes and one 

 liter of water are permitted to stand together for three or 

 four hours. The expressed and filtered fluid, the potato- 

 water, is sterilized in the autoclave for an hour at a tem- 

 perature of 1 10 C. (230 F.), or for fifteen or twenty 

 minutes at a temperature of 120 C. (248 F.), and then 

 instead of the bouillon is used for the preparation of the 

 gelatin. To prepare malt-gelatin, the beer-wort or infusion 

 of malt that can be obtained in any brewery is sterilized, 

 instead of potato-water, and is then further treated in the 

 same way as the bouillon. The only difference, as com- 

 pared with the preparation of simple gelatin, is observed in 

 the alkalinization, only so much normal sodium hydroxid 

 being added as will restore the original feebly acid reaction 

 of the potato-water or of the malt-infusion respectively. 



For the purpose of cultivating typhoid-bacilli directly 

 from the feces, Eisner has devised a modification of potato- 

 gelatin by the addition ot one per cent, of potassium iodid. 

 It is best to add, by means oi a sterile pipet, ^ cu. cm. of 

 a germ-free twenty per cent, solution of potassium iodid to 

 ten cu. cm. of potato-gelatin directly before use. 



(c) Preparation of Agar-agar. Instead of gelatin, from 

 1.2 to 1.5 per cent of agar-agar is added to the peptone- 

 sodium-chlorid meat-infusion. Agar-agar is a vegetable 

 gelatin derived from Japanese and East Indian seaweed. 

 It is best to employ for this purpose powdered agar-agar 

 instead of that in strips, as the latter require a longer time 

 for their solution. After the addition of from five to ten 

 grams of gum arabic, to cause the agar to adhere to the 

 surface of the glass, and after solution of the mixture in the 

 steam-chamber for two or three hours, the fluid is rendered 

 alkaline in the usual manner, the white of an egg is added, 

 heat is again applied for one hour, and the solution is fil- 

 tered. As the agar undergoes coagulation at 39 C. 

 (102.2 F.), the filtering naturally can not be undertaken 

 6 



