CUL TI VA TION OF BA CTERrA . 1 9 1 



cool the liquid to about 6o° C, then add the water mixed 

 with the white of an egg, and then boil again for half an 

 hour, and filter. 



If a double boiler is preferred, it is well to fill the outer 

 boiler with a saturated solution of calcium chlorid, as 

 suggested by Wilson. The boiling-point of this solu- 

 tion is so high that the gelatin or agar-agar in the inner 

 receptacle will boil vigorously. The solution of calcium 

 chlorid can be used again and again, water being added 

 to replace that which evaporates. 



If the filter-paper is of good quality, properly folded 

 (pharmaceutical filter), wet with boiling water, and if the 

 gelatin is properly dissolved, the whole quantity should 

 pass through before cooling too much. Should only half 

 go through before cooling, the remainder must be re- 

 turned 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 un- 

 coagulated albumin, the whole mass can be boiled again. 

 The finished gelatin is at once distributed into sterilized 

 tubes and then sterilized like the bouillon by the frac- 

 tional method. The sterilization can also be satisfactorily 

 performed by the use of the autoclave at iio°-ii5° C. for 

 15 minutes. 



Of course, the gelatin or any other culture-medium can 

 be kept en masse indefinitely, but should a contaminating 

 micro-organism accidentally enter, the whole quantity 

 will be spoiled ; if, on the other hand, it is kept in tubes, 

 several of them may be lost without much inconvenience. 

 Under proper precautions of sterilization and protection 

 it should all keep well. 



Agar-agar. — Agar-agar is the commercial name of a 

 Ceylonese sea-weed which dissolves .in boiling water with 

 resulting thick jelly when cold. The jelly, which solidi- 

 fies between 40 and 50 C, cannot again be melted ex- 

 cept by the elevation of its temperature to the boiling- 



