PREPARATION OF CULTURE MEDIA 13 



organisms present. Agar is somewhat difficult to prepare 

 unless the practitioner has an autoclave, and may be bought 

 with advantage. But the following method is not very diffi- 

 cult, and, as agar is perhaps the most generally useful of all 

 media, should be learnt : 

 Requisites. I. Broth. 



2. Agar-agar. This should be cut up into very small pieces 

 with a pair of scissors, or may be bought in powder, which 

 greatly facilitates its solution. 



3. Solution of acetic acid (glacial acetic acid, 2 to 4 c.c. ; 

 water, 500 c.c.). 



4. A large beaker. 



5. Other apparatus and materials as for gelatin. 

 Method. Weigh out 2 grammes of agar to each 100 c.c. of 



broth to be used, and soak it in the dilute acetic acid for a 

 quarter of an hour. Now strain off the acid and wash the 

 agar in water until a small piece does not redden blue litmus- 

 paper when pressed upon it. Place the broth in a glass 

 beaker and add the agar. Now place the beaker upon a piece 

 of wire gauze on a tripod stand, and apply a small Btinsen 

 flame or spirit-lamp; this must be placed so that the flame 

 impinges on a point not far from the side of the beaker. As 

 the fluid is heated it will rise, and a continual circulation will 

 take place, so that the fragments will not stick to the bottom 

 and cause it to crack. When all is dissolved, the hot liquid 

 must be carefully neutralized. It is then allowed to cool to 

 about 50 C, and the white of an egg added for each 500 c.c. 

 of fluid and mixed in thoroughly by being stirred with a glass 

 rod. The whole is then placed in a steamer for an hour, at 

 the end of which time the albumen should be completely 

 coagulated. The beaker and its contents are then allowed to 

 cool gradually, so that the coagulum (retaining all solid par- 

 ticles) may settle to the bottom before coagulation is com- 

 plete. Perhaps the best method of accomplishing this is to 

 place it in the oven (taking care that the temperature does not 

 exceed 100 C.) after the fire has been raked out at night. In 

 the morning the mass will be found to have solidified, and 

 there will be a coagulum at the bottom. The beaker is then 

 inverted and the mass "turned out" just as a cook turns out 

 a jelly, and the sediment is cut off with a sharp knife. This 

 avoids filtration, which is very troublesome. 



