50 



A MANUAL OF BACTERIOLOGY 



becomes neutral or slightly alkaline is easily ascer- 

 tained by the ordinary test-papers (litmus and tur- 

 meric). It is essential to neutralise any acids, 

 because they are well known to interfere with the 

 growth of many microbes. The extract so obtained 

 is strained through fine linen, and finally filtered 

 through Swedish filter paper. If the filtrate is still 



acid, add a little more 

 sodium carbonate solu- 

 tion ; remove the fat by 

 skimming; add distilled 

 water to make up to the 

 original bulk; and again 

 filter (by means of a hot- 

 water filter, Fig. 19) into 

 a large storage flask or 

 into sterilised test-tubes 

 provided with sterilised 

 plugs of cotton -wool. 

 These vessels and their 

 contents are then heated 

 in the steam steriliser 

 for half-an-hour on each 

 of three successive days. 

 Sometimes the beef ex- 



ria. iy. HOT-WATER *ILTEB. 



tract or bouillon is modi- 

 fied by the addition of other materials. Dr. P. Miquel 

 adds common salt in such proportions as to make 

 a 0'5 per cent, solution. MM. Eoux and Nocard 

 add glycerine to the bouillon before it is finally 

 sterilised. This glycerine-bouillon is an excellent 

 medium for the growth of Bacillus tuberculosis. 



