TEXT-BOOK OF BACTERIOLOGY. 83 



forms an almost colorless mass, of glassy transparency and jelly- 

 like consistency, which when protected from drying 1 up may be 

 preserved a long 1 time without deterioration. In consequence of 

 these properties we can employ gelatin, both in its liquid and in 

 its solid forms, for our culture media. A comparatively small 

 amount of raw gelatin suffices to change our liquid media into 

 solid substances., The more of it used the more solid is the new 

 combination, and the better it is able to withstand the softening 

 influences of heat and other agents. 



We generally use a bouillon with 10$ of gelatin, a mixture which 

 gives the desired degree of solidity without causing too much diffi- 

 culty in its preparation or in its use. 



The way of making this culture solution is as follows : 



The "meat-water" is prepared precisely as for the bouillon 

 already mentioned. Add \% of peptone, \$> of common salt, and in 

 addition to these 10$ of gelatin. 



That is to say, add to the 1,000 grams of meat-water 10 grams 

 of peptone, 5 grams of salt, and 100 grams solid gelatin. 



This mixture is to be well shaken in a large flask, in order to 

 distribute the peptone, and then heated for about half an hour, 

 i.e., till the gelatin is fully dissolved, over an open flame or in the 

 water bath or steam generator. It should not be heated more 

 than this, since otherwise a premature coagulation of the albu- 

 minous matter might easily take place. 



Next follows the neutralization, for gelatin has a decidedly acid 

 reaction. Concentrated solution of carbonate of soda is added till 

 the blue litmus paper is no longer reddened, but the red appears 

 slightly blue; an operation which is frequently a good test of pa- 

 tience and requires a good deal of test paper. To get rid of the 

 coagulable albuminous matter boil for about another hour, at the 

 end of which time the liquid is found to have become clear and some 

 of the albuminous matter is found floating on the surface as a dirty 

 gray scum, while some also has sunk to the bottom. 



Next comes the filtering. A folded filter, such as is used in 

 chemical experiments, is put into a glass funnel and slightly moist- 

 ened with distilled water; then the hot mixture is carefully and 

 slowly poured through it. Avoid pouring in too large quantities at 

 once; the gelatin would otherwise cool in the funnel and could not 

 run through. In order to avoid this difficulty there exist so-called 

 " hot-water funnels." A glass funnel is surrounded on all sides with 

 a covering of copper; between the glass and copper is a closed 

 space which is filled with water. Round the outside copper wall 

 runs a perforated metal tube which can be placed in connection 



