PREPARATION OF CULTURE MEDIA 15 



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 Bunsen 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 particles) may 

 settle to the bottom before coagulation is complete. 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. 



An alternative method is to filter the melted jelly through 

 moistened filter-paper. It is necessary to keep flask and funnel in 

 a steamer (the water of which is kept boiling vigorously) during 

 the whole process, or the jelly will solidify in the outflow tube of 

 the funnel. Or it may be filtered through a double thickness of 

 ordinary surgical lint (non-medicated). It runs through this very 

 quickly, and the funnel need not be kept hot. The resulting 



