PERMANENT CULTURES 63 



on the opposite, after pushing aside the cover-glass, a 

 similar quantity of caustic potash, so that the two fluids 

 mix when the object has been brought into the right 

 position. 



Hens' eggs seem to afford a suitable medium for anae- 

 robic cultures. This kind of cultivation, which is recom- 

 mended by Hiippe and Heim, has been described more in 

 detail above (see p. 50). 



In cultivating obligate anaerobes the materials used in 

 the investigation must, according to Kitisato, be previously 

 heated, in order to remove by this means the facultative 

 anaerobes. 



Permanent cultures. To preserve cultivations of bacteria 

 so that they can be examined at any time, evaporation of 

 the moisture contained in the nutrient medium must be 

 prevented, as well as all possibility of contamination, and 

 consequently the vessels must be hermetically closed. Krai 

 punches cylinders out of boiled potatoes, cuts them into 

 discs, and places them in round glass boxes, the covers 

 of which are tightly ground on and the channels in them 

 filled with glycerine. After inoculation they are closed 

 germ-tight with paraffin and spirit varnish. Cultures may 

 also be preserved in test-tubes hermetically sealed by 

 melting the glass. 



Prausnitz pours an aqueous solution of gelatine con- 

 taining 1 per cent, of carbolic or 5 per cent, of acetic acid 

 over thrust cultures placed in iced water, and then closes 

 them with corks and seals them. 



By Duclaux's method the cultures of bacteria are en- 

 closed in the small tubes used to contain lymph. 



Jacobi first exposes the gelatine which contains the 

 colonies, and which has been spread out in the thinnest 

 possible layer, to the action .of 1 per cent, bichromate of 

 potash for from one to three days in the presence of light, 



