14 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Agar, or Agar- Agar, is the name given to the dried strips of a 

 Japanese seaweed. It forms a jelly which differs from that con- 

 taining gelatin in that it melts at a higher temperature ; nutrient 

 agar, as used in the laboratory, melts just below the boiling-point 

 of water and sets at about 40° C. This is an advantage in the 

 cultivation of most pathogenic bacteria, for these grow (as a rule) 

 best at or near the temperature of the body, the temperature to 

 which they are exposed under natural circumstances ; and at this 

 temperature gelatin would melt. 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 

 difficult, 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. 



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 for 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 the steamer for an 

 hour, at the end of which time the albumin 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 



