14 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



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" vigor- 

 ously) 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 medium is not absolutely clear, but 

 sufficiently so for most purposes. 



The agar is again melted and placed in test-tubes ; these are 

 sterilized on three successive days and allowed to set in a 

 sloping position. For certain purposes glucose, glycerin, etc., 

 are added to the agar. The addition should be made to the 

 melted medium just before the final sterilization. 



Douglas's agar has been described previously. 



SOLIDIFIED BLOOD-SERUM* is best purchased ready for use 

 from a good laboratory. It is used chiefly in the diagnosis 

 of diphtheria by the examination of " swabs " from the throat. 

 It can be prepared without difficulty from the sterile (chloro- 

 formed) blood-serum now obtainable commercially. Place 

 about 5-8 c.c. in sterile tubes, place them in a sloping position 

 in a water-bath (a shallow tin tray will do), heat it to 80 C. for 

 one hour, then to 100 C. until the serum is coagulated. The 

 temperature will drive off the chloroform added as a pre- 

 servative. 



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 f inch in diameter (if 

 you are using f-inch 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. Then cut each 

 cylinder in half by a cut running obliquely from end to end ; 

 the shape of each half should be exactly like that of the medium 

 in a sloped gelatin tube. Place the halves in a large vessel of 

 tap-water and allow them to soak all night ; it is a good plan 

 to use running water if possible. 



After this has been done place each half (base downwards) 

 in a test-tube, having previously inserted a small mass of 

 absorbent cotton-wool and enough water to saturate it. Plug 



