PREPARATION OF CULTURE MEDIA. 21 



and is best purchased ready for use from a good 

 laboratory. For clinical purposes it is chiefly used in 

 the preparation of the diphtheria bacillus, for which, 

 however, it may be replaced by the following medium, 

 which is more easy to prepare. 

 ASCITIC AGAR : Requisites : 



1. Clear ascitic, pleuritic, or hydrocele fluid. 



2. A 10 per cent, solution of caustic potash. 



3. Glycerine. 



4. Other materials and apparatus as for agar. (Broth 

 is not required, its place being taken by the pathological 

 exudate). 



Method. Measure the fluid and add to it 2 c.c. of the 

 potash solution for each 100 c.c. used. Mix thoroughly, 

 pour a little into a test-tube, and heat. If the fluid 

 does not coagulate proceed with the manufacture of the 

 medium in the manner described below. If it coagu- 

 lates add a little more potash, mix, and again test. 

 The addition of the potash 'must be repeated until no 

 coagulum forms on heating, all the albumen being 

 converted into uncoagulable alkali-albumen. 



The remaining steps of the process are the same as 

 for ordinary agar, except that the neutralisation is 

 omitted and 6 c.c. of glycerine per 100 c.c. of the 

 medium is added whilst the agar is being dissolved. 



POTATO tubes are in occasional use, and are easy to 

 prepare. The process is as follows : Take large and 

 sound potatoes and scrub them thoroughly with a nail- 

 brush under the tap. Peel them deeply enough to 

 remove the eyes completely. Then cut them into 

 cylinders a little less than in. in diameter (if you are 

 using f in. test tubes) and as long as possible ; this is 

 best done by means of a cork-borer, but they may be 

 shaped by means of a knife if this is not at hand. 



