ii.] PREPARATION OF CULTURE MATERIAL. 11 



organism, nevertheless possesses full pathogenic power, then it 

 is logical to say that this pathogenic property rests with the 

 organism. For this and other reasons it is of essential 

 importance to be 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 : 



1. 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, and the 

 broth well neutralised by adding liquor potassse 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, previously overheated (see below), into flasks 

 previously sterilised (see below). If the broth is not clear 

 after once filtering 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 white of an egg. The now clear fluid is filtered again. 

 The flasks which receive the broth are well plugged with 

 sterilised cotton-wool (see below). In this state the flask is 

 placed over a Bunsen burner (Fig. 2) on a wire netting and 

 boiled for half an hour or more ; during the boiling the cotton- 

 wool plug is lifted out for half its length. The flask ought 

 not to contain more broth than about one-half or two-thirds of 

 its volume, to prevent the broth from rising too much and 

 wetting the plug. When turning off the flame the plug is 

 pushed down so as fully to plug the neck and mouth of the 

 flask ; a beaker with sterile cotton-wool cap is placed over 



