226 



Then 2«/o agar is added. Next day the mixture i^ liquefied, 

 filtered through cotton wool, distributed in 600c.c. flasks and 

 sterilised in the steamer (100°) on 3 consecutive days. This 

 brings down the Ph- value to roughly 7.2 (determined colo- 

 rimetrically in broth without agar, sterilised in the same way). 

 The agar ought not to be autoclaved as it has been shown that 

 this renders it a distinctly worse basic medium for the hae- 

 moglobinophilic bacteria. 



It may sometimes nappen that the finished agar has not 

 acquired just the reaction desired; an appropriate amount of 

 sodium hydroxide or hydrochloric acid can then be added after 

 sterilisation. 



It is slated fairly unanimously by a number of authors 

 that the optimum reaction for Pfeiffer's bacillus lies between 

 Ph- 7.2—7.5 (nearest the first value). 



For controlling the reaction of the finished medium I found 

 it convenient to use azolitmin paper which I prepared myself 

 from the instructions of Henriques & Sorensen (I used double 

 the amount of azolitmin however), and also a 1% solution of 

 a-naphlholphtalein (Sorensen & Palitsch) in dilute alcohol. 

 In phosphate mixtures (Sorensen) of Ph- 6.8 azolitmin paper 

 reacts neutral, that is to say, retains its violet colour, while 

 «-naphthol-phthalein (in not too small quantity) shows a sharp 

 change of colour (a gray colour as a transition from weak 

 yellowish-brown to deep bluish-green) at Ph- 7.4. With the 

 aid of these indicators in conjunction with phenolphthalein 

 (which begins to change colour at Ph- 8.0) we can, within a 

 range of from about 6.8—8.0, judge the reaction of the medium 

 with a precision which will certainly be quite sufficient in most 

 cases for ordinary bacteriological purposes. They can be em- 

 ployed in many cases where a more exact colorimetric measure- 

 ment would be impossible or very difficult (very turbid and 

 coloured media etc.). 



It has been found that broth or agar media without any 

 ascitic fluid have a reaction suitable for Pfeiffer's bacillus 

 and many other bacteria when they react strongly alkaline 

 to azolitmin paper without changing the yellow colour of 

 a-naphtholphthalein. Ascitic agar however should react neu- 

 tral or slightly alkaline to azolitmin paper. These facts apply 



