90 METHODS OF CULTIVATION OF BACTERIA 



Warm to about 40 C., and removing cotton- wool plug from 

 culture take a little of the preserving fluid in a pipette and 

 allow to run gently over surface of medium in tube. Place in 

 such a position that a thin layer of the preserving medium 

 remains completely covering the growth and the surface of 

 culture medium. The gelatin is now allowed to solidify. Add 

 three or four drops of strong formalin to the tube, and fill up to 

 within a quarter of an inch of the top of the tube with the 

 following fluid : 



(2) Thymol water (saturated in cold) . . . 100 c.c. 

 Glycerin . . . . . . . 20 c.c. 



Acetate of potash ...... 5 .grms. 



Cover top of tube with a small piece of paper so as to keep out 

 dust, allow to stand for a day or two so that small air-bells may 

 rise to the surface. 



To seal tube, pour melted paraffin gently on to the surface 

 of fluid up to near the top of tube ; allow to solidify. Cover 

 paraffin with layer of alcoholic orange shellac cement; allow 

 this to set, and repeat until the cement becomes level with top 

 of test-tube. When the cement is set, a few drops of black 

 lacquer are put on, and a circular cover-glass of about the 

 same diameter as the mouth of tube is placed so as completely 

 to seal it. 



(b) The following method is useful for preserving plate cultures : 

 Instead of making the cultures in Petri's capsules, use ordinary 

 watch-glasses. The watch-glass is sterilised in a Petri's capsule, 

 and the inoculated medium is poured out into the watch-glass, 

 allowed to solidify in the usual way, and left in the Petri's 

 capsule until the colonies of growth have developed. The 

 watch-glass is now removed from capsule, and a layer of the pre- 

 serving gelatin medium (1) (p. 89), to which have been added a few 

 drops of strong formalin, is allowed to spread over the surface 

 of the culture medium. When the layer is solidified the watch- 

 glass is filled up with the same, and a clean square or oblong 

 piece of glass (which of course should be of slightly larger 

 diameter than the watch-glass) is now carefully placed over 

 watch-glass, care being taken that no air-bells are formed. The 

 edge of watch-glass should be closely applied to the glass cover, 

 and left in position until the gelatin has solidified. The super- 

 fluous gelatin is now removed, and the glasses sealed first with 

 the orange shellac cement, then with black lacquer. It is now r 

 finished off by using a circular mask of suitable size. 



The various kinds of solid media used in the cultivation of 



