II] PREPARATION OF CULTURE MATERIAL 27 



able to carry on successive cultivations of one and the 

 same organism without any accidental contamination or 

 admixture — i.e. it is necessary to carry on pure cultivations. 



ARTIFICIAL CULTIVATION MEDIA. 



A. — Fluids. 



As fluid nourishing material the following are used with 

 preference : — 



I. Broth made from Meat— pork, beef, rabbit, chicken. — The 

 connective tissue and fat are first cut out from the fresh meat 

 — in the case of rabbit or chicken the whole animal without 

 head or viscera is used — and then placed in water and boiled. 

 Generally for each pound half an hour's good boiling is 

 allowed. With regard to the quantity of water, each pound 

 of meat ought to yield ultimately at least one pint of broth. 

 When boiled, the broth is allowed to stand, the fat is skimmed 

 off, arid the broth well neutralised, or even made faintly 

 alkaline by adding liquor potassee, or, better still, carbonate 

 of sodium. 



The fresher the, meat the less acid.(sarcolactic acid) is in 

 the broth before neutralisation. The broth is then filtered 

 through a filter ^ into flasks previously sterilised (see below). 

 As a rule beef broth is clear, but if not it is filtered again. 

 If not clear then, it is allowed to stand for several hours. 

 A fine sediment is found at the bottom of the vessel, and 

 from this the clear supernatant fluid is decanted into a 

 sterilised vessel. The broth, if not clear after the first 

 filtering, can be cleared by boiling it with the broken shell 



' Unless otiierwise slated all filtration is carried out by means of 

 folded Swedish filter paper. 



