AGAR-AGAR. 39 



tin is very liable to burn. If the gelatin is soaked in warm 

 water before it is added to the bouillon, it will dissolve much 

 more rapidly when boiled. Neutralize and cool to 60 C. 

 Take the whites of two eggs and beat them up thoroughly 

 with a little water ; add this to the gelatin mixture and stir 

 well. Boil for ten minutes, stirring constantly. The whites 

 of the eggs clear the solution by embracing most of the im- 

 purities in their coagulum. 



In order to hasten the filtration the solution is filtered 

 while it is hot. The portion that is not poured on the filter 

 at once should be kept hot over a low flame until ready to be 

 used. Gelatin may be filtered either through filter-paper, a 

 thick layer of cotton, or a double layer of close-woven cheese- 

 cloth. A very rapid way is to place the filter-paper in the 

 funnel, and then spread over the top of the funnel a layer of 

 cheese-cloth. As the solution is poured on the cheese-cloth 

 this catches the coarser particles of coagulated albumin and 

 prevents clogging of the filter-paper. The gelatin should be 

 clear. Excessive alkalinity clouds the medium. Prolonged 

 boiling prevents solidification. 



After filtration the gelatin is decanted into test-tubes, 10 c.c. 

 in each tube, and sterilized for fifteen minutes on each of three 

 successive days in the steam sterilizer. After the last ster- 

 ilization the tube should be placed in the upright position, so 

 that the solidified medium will have a flat, horizontal surface. 

 In warm weather it is necessary to add more than 10 per 

 cent, of gelatin in order to keep the medium solid. Gelatin 

 can be modified by the addition of glucose, glycerin, or blood- 

 serum. 



Agar-agar : This is a Ceylonese sea-weed having a very 

 high melting-point, and is suitable for cultures which must be 

 subjected to the incubator temperature for a long time. Un- 

 like gelatin, it cannot be liquefied repeatedly without spoiling 

 it. Agar-agar medium is the hardest to prepare. Its prepa- 

 ration is extremely tedious, and unless the greatest care is 

 exercised it is very apt to spoil. It requires constant watch- 

 ing. Its formula is the same as that for beef-tea, plus 1 per 

 cent, of agar-agar : 



