102 



BACTERIOLOGY. 



paper becomes faintly blue. If it has been made too alkaline, it can 

 be neutralised by the addition of lactic acid. 



Finally, the mixture is heated for an hour in the water-bath. 

 Ten minutes before the boiling is completed, the white of an egg 

 beaten up with the shell is added, and the liquid is then filtered 

 while hot. For the filtration, the hot- water apparatus (Fig. 31) 

 can be used with advantage, furnished with a filter of Swedish 



paper, which may be conveniently 

 made in the following way : 



About eighteen inches square 

 of the best and stoutest filter paper 

 is first folded in the middle, 'and 

 then creased into sixteen folds. The 

 filter is made to fit the glass funnel 

 by gathering up the folds like a fan, 

 and cutting off the superfluous part. 

 The creasing of each fold should be 

 made firmly to within half an inch 

 of the apex of the filter, which part 

 is to be gently inserted into the 

 tube of the funnel. To avoid 

 bursting the filter at the point, the 

 broth, when poured out from the 

 flask, should be diiected against 

 the side of the filter with a glass 

 rod. During filtration the funnel 

 should be covered over w r ith a 

 circular plate of glass, and the pro- 

 cess of filtration must be repeated, 

 if necessary, until a pale, straw- 

 coloured, perfectly transparent 

 filtrate results.- 



The sterilised test-tubes are filled to about a third of their depth 

 by pouring in the gelatine carefully and steadily, or by employing a 

 small sterilised glass funnel. The object of this care is to prevent 

 the mixture touching the part of the tube with which the plug 

 comes into contact ; otherwise, when the gelatine sets, the cotton- 

 wool adheres to the tube and becomes a source of embarrassment in 

 subsequent procedures. As the tubes are filled they are placed in 

 the test-tube basket, and must then be sterilised. They are either 

 lowered into the steam steriliser, when the thermometer indicates 

 100 0., for twelve minutes for four or five successive days, or they 



FIG. 31. HOT-WATER FILTERING 

 APPARATUS. 



