no PATHOGENIC BACTERIA. 



the whole mass can be boiled again. The finished gel- 

 atin is at once distributed into sterilized tubes, and then 

 sterilized like the bouillon by the fractional method. 



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 

 Japanese sea- weed which dissolves in boiling water with 

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

 fies between 30 and 40 C., cannot again be melted ex- 

 cept by the elevation of its temperature to the boiling- 

 point, so that this culture-medium, which is nearly trans- 

 parent, is almost as useful as gelatin. In addition to its 

 readiness to liquefy and solidify, it is sufficiently firm 

 to allow of the incubation-temperature i. e. 37 C. at 

 which gelatin is always liquid, and no better than bouillon. 



The preparation of this medium is generally described 

 in the text-books as one u requiring considerable patience 

 and much waste of filter- paper. " In reality, it is not dif- 

 ficult if a good heavy filter-paper be obtained and no 

 attempt be made to filter the solution until the agar-agar 

 is perfectly dissolved. It is prepared as follows : To 1000 

 c.cm. of bouillon made as described above, preferably of 

 meat instead of beef-extract, 10 grams of agar-agar are 

 added. The mixture is boiled for an hour, or, if possible, 

 two. At the end of the first hour it is cooled to about 

 60 C., and after neutralization, which may not be neces- 

 sary if the bouillon was neutral, an egg beaten up in 

 water is added, and the liquid is boiled again until the 

 egg is entirely coagulated. The reaction of the agar-agar 

 should be neutral rather than alkaline, as, for an un- 

 known reason, alkalinity seems to interfere slightly with 

 filtration. 



After the boiling, which should be brisk, has caused 



